diff --git a/books/free-programming-books-langs.md b/books/free-programming-books-langs.md
index 081848307..12d93b8a2 100644
--- a/books/free-programming-books-langs.md
+++ b/books/free-programming-books-langs.md
@@ -1587,7 +1587,7 @@ Books on general-purpose programming that don't focus on a specific language are
 * [Mathematica® programming: an advanced introduction](http://www.mathprogramming-intro.org) - Leonid Shifrin
 * [Power Programming with Mathematica](http://mathematica.stackexchange.com/questions/16485/are-you-interested-in-purchasing-david-wagners-power-programming-with-mathemat/22724) - David B. Wagner
 * [Stephen Wolfram's The Mathematica Book](http://reference.wolfram.com/legacy/v5_2/)
-* [Vector Math for 3d Computer Graphics](http://chortle.ccsu.edu/VectorLessons/index.html)
+* [Vector Math for 3d Computer Graphics](http://chortle.ccsu.edu/VectorLessons/index.html) (CC BY-NC)
 * [Wolfram Mathematica Product Training: Wolfram U](https://www.wolfram.com/wolfram-u/catalog/product-training/mathematica/)