diff --git a/books/free-programming-books-subjects.md b/books/free-programming-books-subjects.md
index fc8f61700..a3e97e77a 100644
--- a/books/free-programming-books-subjects.md
+++ b/books/free-programming-books-subjects.md
@@ -114,7 +114,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Open Data Structures: An Introduction](http://opendatastructures.org) - Pat Morin
 * [Planning Algorithms](http://planning.cs.uiuc.edu)
 * [Problems on Algorithms (Second Edition)](http://ianparberry.com/books/free/license.html) - Ian Parberry (use form at bottom of license)
-* [Purely Functional Data Structures (1996)](http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf) - Chris Okasaki (PDF)
+* [Purely Functional Data Structures (1996)](https://web.archive.org/web/20210917054102/http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf) - Chris Okasaki (PDF) *(:card_file_box: archived)*
 * [Sequential and parallel sorting algorithms](http://www.inf.fh-flensburg.de/lang/algorithmen/sortieren/algoen.htm)
 * [Text Algorithms](http://igm.univ-mlv.fr/~mac/REC/text-algorithms.pdf) (PDF)
 * [The Algorithm Design Manual](http://www8.cs.umu.se/kurser/TDBAfl/VT06/algorithms/BOOK/BOOK/BOOK.HTM)
@@ -322,7 +322,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [A Brief Introduction to Neural Networks](http://www.dkriesel.com/en/science/neural_networks)
 * [A Comprehensive Guide to Machine Learning](https://www.eecs189.org/static/resources/comprehensive-guide.pdf) - Soroush Nasiriany, Garrett Thomas, William Wang, Alex Yang (PDF)
 * [A Course in Machine Learning](http://ciml.info/dl/v0_9/ciml-v0_9-all.pdf) (PDF)
-* [A First Encounter with Machine Learning](https://www.ics.uci.edu/~welling/teaching/ICS273Afall11/IntroMLBook.pdf) (PDF)
+* [A First Encounter with Machine Learning](https://web.archive.org/web/20210420163002/https://www.ics.uci.edu/~welling/teaching/ICS273Afall11/IntroMLBook.pdf) - Max Welling (PDF) *(:card_file_box: archived)*
 * [A Selective Overview of Deep Learning](https://arxiv.org/abs/1904.05526) - Fan, Ma, and Zhong (PDF)
 * [Algorithms for Reinforcement Learning](https://sites.ualberta.ca/~szepesva/papers/RLAlgsInMDPs.pdf) - Csaba Szepesvári (PDF)
 * [An Introduction to Statistical Learning](https://web.stanford.edu/~hastie/ISLR2/ISLRv2_website.pdf) - Gareth James, Daniela Witten, Trevor Hastie and Robert Tibshirani (PDF)
@@ -334,7 +334,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Dive into Deep Learning](http://d2l.ai)
 * [Explorations in Parallel Distributed Processing: A Handbook of Models, Programs, and Exercises](https://web.stanford.edu/group/pdplab/pdphandbook) - James L. McClelland
 * [Foundations of Machine Learning, Second Edition](https://mitpress.ublish.com/ereader/7093/?preview=#page/Cover) - Mehryar Mohri, Afshin Rostamizadeh, Ameet Talwalkar
-* [Free and Open Machine Learning](https://freeandopenmachinelearning.readthedocs.io/en/latest/index.html) - Maikel Mardjan
+* [Free and Open Machine Learning](https://nocomplexity.com/documents/fossml/) - Maikel Mardjan (HTML)
 * [Gaussian Processes for Machine Learning](http://www.gaussianprocess.org/gpml/)
 * [IBM Machine Learning for Dummies](https://www.ibm.com/downloads/cas/GB8ZMQZ3) - Judith Hurwitz, Daniel Kirsch
 * [Information Theory, Inference, and Learning Algorithms](http://www.inference.phy.cam.ac.uk/itila/)
@@ -407,7 +407,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Complex Analysis](https://people.math.gatech.edu/~cain/winter99/complex.html) - George Cain
 * [Computational and Inferential Thinking. The Foundations of Data Science](https://www.inferentialthinking.com)
 * [Computational Geometry](http://web.mit.edu/hyperbook/Patrikalakis-Maekawa-Cho/)
-* [Computational Mathematics with SageMath](http://sagebook.gforge.inria.fr/english.html) - Paul Zimmermann et al.
+* [Computational Mathematics with SageMath](https://www.sagemath.org/sagebook/) - Paul Zimmermann, Alexandre Casamayou, Nathann Cohen, Guillaume Connan, et al. (PDF)
 * [Concepts & Applications of Inferential Statistics](http://vassarstats.net/textbook/)
 * [Convex Optimization](https://web.stanford.edu/~boyd/cvxbook) - Stephen Boyd and Lieven Vandenberghe
 * [Coordinate Geometry (1911)](http://djm.cc/library/Coordinate_Geometry_Fine_Thompson_edited03.pdf) - Henry Buchard Fine and Henry Dallas Thompson (PDF)
@@ -437,8 +437,8 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Kalman and Bayesian Filters in Python](https://github.com/rlabbe/Kalman-and-Bayesian-Filters-in-Python)
 * [Knapsack Problems - Algorithms and Computer Implementations](http://www.or.deis.unibo.it/knapsack.html) - Silvano Martello and Paolo Toth
 * [Lecture Notes of Linear Algebra](http://home.iitk.ac.in/~psraj/mth102/lecture_notes.html) - Dr. P. Shunmugaraj, IIT Kanpur (PDF)
+* [Lecture Notes on Linear Algebra](https://home.iitk.ac.in/~arlal/book/LA_Solution_Dec20.pdf) - Dr. Arbind K Lal, Sukant Pati (PDF) (:construction: *in process*)
 * [Lies, Damned Lies, or Statistics: How to Tell the Truth with Statistics](https://www.poritz.net/jonathan/share/ldlos.pdf) - Jonathan A. Poritz (PDF)
-* [Linear Algebra](http://home.iitk.ac.in/~arlal/book/la.pdf) - Dr. Arbind K Lal, Sukant Pati (PDF) (:construction: *in process*)
 * [Linear Algebra](https://www.math.ucdavis.edu/~linear/linear-guest.pdf) (PDF)
 * [Linear Algebra](http://joshua.smcvt.edu/linearalgebra/) - Jim Hefferon
 * [Linear Algebra Done Wrong](https://www.math.brown.edu/streil/papers/LADW/LADW.html) - Sergei Treil
@@ -456,7 +456,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Non-Uniform Random Variate Generation](http://luc.devroye.org/rnbookindex.html) - Luc Devroye (PDF)
 * [Notes on Diffy Qs](https://www.jirka.org/diffyqs/) - Jiří Lebl
 * [Number Theory](https://github.com/holdenlee/number-theory) - Holden Lee MIT
-* [Number Theory: In Context and Interactive](http://math.gordon.edu/ntic/) - Karl-Dieter Crisman
+* [Number Theory: In Context and Interactive](https://web.archive.org/web/20210711231452/https://math.gordon.edu/ntic/) - Karl-Dieter Crisman
 * [Odds and Ends: Introducing Probability & Decision with a Visual Emphasis](https://jonathanweisberg.org/vip/) - Jonathan Weisberg
 * [Online Statistics Education](http://onlinestatbook.com) - David Lane
 * [OpenIntro Statistics](https://www.openintro.org/stat/textbook.php) - David M. Diez, Christopher D. Barr, Mine Çetinkaya-Rundel
@@ -514,20 +514,19 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Digital Signal Processing in Python](https://greenteapress.com/wp/think-dsp) - Allen B. Downey
 * ["DYNAMIC LINKED LIBRARIES": Paradigms of the GPL license in contemporary software](http://www.lulu.com/shop/http://www.lulu.com/shop/luis-enr%C3%ADquez-a/dynamic-linked-libraries-paradigms-of-the-gpl-license-in-contemporary-software/ebook/product-21419788.html) - Luis A. Enríquez
 * [Essential Image Optimization](https://images.guide) - Addy Osmani
-* [Foundations of Programming](http://openmymind.net/FoundationsOfProgramming.pdf) (PDF)
+* [Foundations of Programming](https://openmymind.net/FoundationsOfProgramming.pdf) - Karl Seguin (PDF)
 * [Getting Real](https://basecamp.com/books/getting-real) - Basecamp, 37signals ([HTML](https://basecamp.com/gettingreal), [PDF](https://basecamp.com/gettingreal/getting-real.pdf))
 * [Google Maps API Succinctly](https://www.syncfusion.com/ebooks/google_maps_api_succinctly) - Mark Lewin
 * [Hacknot: Essays on Software Development](http://www.lulu.com/shop/ed-johnson/hacknot-essays-on-software-development/ebook/product-17544641.html) - Ed Johnson
 * [Hello SDL](https://lazyfoo.net/tutorials/SDL) - Lazy Foo' Productions
 * [High-Performance Scientific Computing](https://andreask.cs.illinois.edu/Teaching/HPCFall2012) (class lectures and slides)
-* [Holm School (You Might Learn Something) - How to Teach Yourself To Be A Computer Scientist for Hamiltons a Month](https://ryheimat.github.io/holm-school) - Ryhen Holm
 * [HoloLens Succinctly](https://www.syncfusion.com/ebooks/hololens_succinctly) - Lars Klint
 * [How Computers Work](http://www.fastchip.net/howcomputerswork/p1.html) - R. Young
 * [How to Become a Programmer](http://softwarebyrob.wpengine.netdna-cdn.com/assets/Software_by_Rob%20_How_to_Become_a%20_Programmer_1.0.pdf) (PDF)
 * [How To Manage Remote Servers with Ansible](https://www.digitalocean.com/community/books/how-to-manage-remote-servers-with-ansible-ebook) - Erika Heidi (PDF, EPUB)
 * [How to Think Like a Computer Scientist](http://openbookproject.net/thinkcs/) - Peter Wentworth, Jeffrey Elkner, Allen B. Downey, and Chris Meyers
 * [Image Processing in C: Analyzing and Enhancing Digital Images](http://homepages.inf.ed.ac.uk/rbf/BOOKS/PHILLIPS/) - Dwayne Phillips
-* [Information Technology and the Networked Economy](http://florida.theorangegrove.org/og/file/49843a6a-9a9d-4bad-b4d4-d053f9cdf73e/1/InfoTechNetworkedEconomy.pdf) (PDF)
+* [Information Technology and the Networked Economy](https://web.archive.org/web/20200731035935/https://florida.theorangegrove.org/og/file/49843a6a-9a9d-4bad-b4d4-d053f9cdf73e/1/InfoTechNetworkedEconomy.pdf) - Patrick McKeown (PDF) *(:card_file_box: archived)*
 * [Introduction to Scientific Programming in C++ and Fortran](https://web.corral.tacc.utexas.edu/CompEdu/pdf/isp/EijkhoutIntroSciProgramming-book.pdf) - Victor Eijkhout (PDF)
 * [IRPF90 Fortran code generator](https://www.gitbook.com/book/scemama/irpf90/details) - Anthony Scemama
 * [Learn Programming](https://progbook.org) - Antti Salonen
@@ -552,7 +551,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Record-Playback Test Automation: Sahi & Selenium IDE: Critical Evaluation of Record-Playback Automation Tools](https://leanpub.com/manualToAutomatedWithSeleniumIDEAndSahi) - Shashikant Jagtap  *(Leanpub account or valid email requested)*
 * [Scientific Programming and Computer Architecture](https://divakarvi.github.io/bk-spca/spca.html) - Divakar Viswanath
 * [Signal Computing: Digital Signals in the Software Domain](http://faculty.washington.edu/stiber/pubs/Signal-Computing/Signal%20Computing.pdf) - Michael Stiber, Bilin Zhang Stiber, Eric C. Larson (PDF)
-* [Small Memory Software](http://www.smallmemory.com/book.html)
+* [Small Memory Software](https://smallmemory.charlesweir.com/book.html) - Charles Weir, James Noble (HTML)
 * [Web Almanac](https://almanac.httparchive.org/static/pdfs/web_almanac_2019_en.pdf) (PDF)
 * [Writing Native Mobile Apps in a Functional Language Succinctly](https://www.syncfusion.com/ebooks/writing_native_mobile_apps_in_a_functional_language_succinctly) - Vassili Kaplan
 
@@ -726,8 +725,8 @@ Kerridge (PDF) (email address *requested*, not required)
 * [How to Write Unmaintainable Code](http://mindprod.com/jgloss/unmain.html)
 * [Kanban and Scrum - making the most of both](http://www.infoq.com/minibooks/kanban-scrum-minibook)
 * [Kanban for skeptics](https://leanpub.com/kanbanforskeptics) - Nick Oostvogels  *(Leanpub account or valid email requested)*
-* [Microservices AntiPatterns and Pitfalls](https://www.oreilly.com/programming/free/files/microservices-antipatterns-and-pitfalls.pdf) (PDF)
-* [Microservices vs. Service-Oriented Architecture](https://www.oreilly.com/programming/free/files/microservices-vs-service-oriented-architecture.pdf) (PDF)
+* [Microservices AntiPatterns and Pitfalls](http://web.archive.org/web/20210205164251/https://www.oreilly.com/programming/free/files/microservices-antipatterns-and-pitfalls.pdf) - Mark Richards (PDF) *(:card_file_box: archived)*
+* [Microservices vs. Service-Oriented Architecture](https://www.oreilly.com/radar/microservices-vs-service-oriented-architecture/) - Mark Richards (HTML)
 * [Migrating to Cloud-Native Application Architectures](https://developers.redhat.com/books/migrating-microservice-databases-relational-monolith-distributed-data/) (email address *requested*) (PDF)
 * [Naked objects](http://downloads.nakedobjects.net/resources/Pawson%20thesis.pdf) - Richard Pawson (PDF)
 * [OAuth - The Big Picture](https://pages.apigee.com/oauth-big-picture-ebook.html) (email address *requested*)