Guide to Discrete Mathematics Book Add (#9628)

* Guide to Discrete Mathematics Book Add

* Add Mathematic book

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

* Update link

* Update books/free-programming-books-subjects.md

Co-authored-by: Leithen <lcrider@cvent.com>

---------

Co-authored-by: Leithen <lcrider@cvent.com>

books/free-programming-books-subjects.md

@@ -503,6 +503,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [Foundations of Constructive Probability Theory](https://arxiv.org/pdf/1906.01803.pdf) - Yuen-Kwok Chan (PDF)
 * [Geometry with an Introduction to Cosmic Topology](https://mphitchman.com) - Michael P. Hitchman
 * [Graph Theory](http://compalg.inf.elte.hu/~tony/Oktatas/TDK/FINAL/)
+* [Guide to Discrete Mathematics](https://core.ac.uk/download/pdf/326762636.pdf) - David Gries, Fred B. Schneider (PDF)
 * [How We Got from There to Here: A Story of Real Analysis](https://milneopentextbooks.org/how-we-got-from-there-to-here-a-story-of-real-analysis/) - Robert Rogers, Eugene Boman
 * [Introduction to Modern Statistics](https://openintro-ims.netlify.app) - Mine Çetinkaya-Rundel, Johanna Hardin (HTML, PDF) (email address required for PDF)
 * [Introduction to Probability](https://math.dartmouth.edu/~prob/prob/prob.pdf) - Charles M. Grinstead, J. Laurie Snell (PDF)