diff --git a/books/free-programming-books-langs.md b/books/free-programming-books-langs.md
index e7f53a84c..5b12434e7 100644
--- a/books/free-programming-books-langs.md
+++ b/books/free-programming-books-langs.md
@@ -527,7 +527,7 @@ Books on general-purpose programming that don't focus on a specific language are
 * [C++ Tutorial](https://www.cprogramming.com/tutorial/c++-tutorial.html) - Alex Allain
 * [CS106X Programming Abstractions in C++](http://web.stanford.edu/class/cs106x/)
 * [Elements of Programming](http://elementsofprogramming.com) - Alexander Stepanov, Paul McJones (PDF)
-* [Essential C++](https://www.programming-books.io/essential/cpp/) - Krzysztof Kowalczyk, StackOverflow Contributors
+* [Essential C++](https://www.programming-books.io/essential/cpp/) - Krzysztof Kowalczyk, StackOverflow Contributors (CC BY-SA)
 * [Financial Numerical Recipes in C++](https://ba-odegaard.no/gcc_prog/recipes/) - Bernt Arne Ødegaard (PDF)
 * [Fundamentals of C++ Programming](https://web.archive.org/web/20191005170118/https://python.cs.southern.edu/cppbook/progcpp.pdf) - Richard L. Halterman (PDF) *(:card_file_box: archived)*
 * [Game Programming Patterns](http://gameprogrammingpatterns.com/contents.html) (HTML)
diff --git a/books/free-programming-books-subjects.md b/books/free-programming-books-subjects.md
index 2f4855826..db8abd3c2 100644
--- a/books/free-programming-books-subjects.md
+++ b/books/free-programming-books-subjects.md
@@ -742,7 +742,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [The Little Book About OS Development](https://littleosbook.github.io) - Erik Helin, Adam Renberg - (PDF, HTML)
 * [The Little Book of Semaphores](https://greenteapress.com/semaphores/) - Allen B. Downey (PDF) (CC BY-NC-SA)
 * [Think OS: A Brief Introduction to Operating Systems](https://www.greenteapress.com/thinkos/index.html) - Allen B. Downey (PDF)
-* [UNIX Application and System Programming, lecture notes](http://www.compsci.hunter.cuny.edu/~sweiss/course_materials/unix_lecture_notes.php) - Stewart Weiss (PDF)
+* [UNIX Application and System Programming, lecture notes](http://www.compsci.hunter.cuny.edu/~sweiss/course_materials/unix_lecture_notes.php) - Stewart Weiss (PDF) (CC BY-SA)
 * [Writing a Simple Operating System from Scratch](https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf) - Nick Blundell (PDF)
 * [Xv6, a simple Unix-like teaching operating system](https://pdos.csail.mit.edu/6.828/2022/xv6.html) - Russ Cox, Frans Kaashoek and Robert Morris (PDF, HTML)