docs(books): solved two issues (#6686)

* docs(books): solved two issues

* docs(books): moved resource

books/free-programming-books-subjects.md

@@ -262,6 +262,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 ### Game Development
 
 * [2D Game Development: From Zero To Hero](https://github.com/Penaz91/2DGD_F0TH) - Daniele Penazzo (HTML, [PDF, EBPUB, Kindle...](https://therealpenaz91.itch.io/2dgd-f0th#download)) (:construction: *in process*)
+* [3D Math Primer for Graphics and Game Development](https://gamemath.com/book/intro.html) - Fletcher Dunn (HTML)
 * [Coding With Minecraft](https://turtleappstore.com/book/) - Al Sweigart
 * [Designing Virtual Worlds](https://mud.co.uk/richard/DesigningVirtualWorlds.pdf) - Richard A. Bartle (PDF)
 * [Game AI Pro](http://www.gameaipro.com) - Steve Rabin
@@ -491,6 +492,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [A Mathematical Theory of Communication](https://archive.org/details/bstj27-4-623) - Claude E.Shannon
 * [Discrete Structures for Computer Science: Counting, Recursion, and Probability](http://cglab.ca/~michiel/DiscreteStructures/) - Michiel Smid
 * [Exploring Math for Programmers and Data Scientists](https://freecontent.manning.com/free-ebook-exploring-math-for-programmers-and-data-scientists/) - Paul Orland
+* [Mathematics for Computer Science](https://courses.csail.mit.edu/6.042/spring18/mcs.pdf) - Eric Lehman, F. Thomson Leighton, Albert R. Meyer (PDF)
 
 
 ### Misc