adding infinite descent into pure mathematics pdf (#6885)

* adding infinite descent into pure mathematics pdf

* corrected alphabetical order

ignore the "An" at the beginning of the title before, and corrected that here

books/free-programming-books-subjects.md

@@ -390,6 +390,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Algebra: An Elementary Text-Book, Part I (1904)](http://djm.cc/library/Algebra_Elementary_Text-Book_Part_I_Chrystal_edited.pdf) - G. Chrystal (PDF)
 * [Algebra: An Elementary Text-Book, Part II (1900)](http://djm.cc/library/Algebra_Elementary_Text-Book_Part_II_Chrystal_edited02.pdf) - G. Chrystal (PDF)
 * [Algebraic Topology](https://pi.math.cornell.edu/~hatcher/AT/ATpage.html) - Allen Hatcher (PDF)
+* [An Infinite Descent into Pure Mathematics](https://infinitedescent.xyz/dl/infdesc.pdf) - Clive Newstead (PDF)
 * [An Introduction to the Theory of Numbers](http://www.trillia.com/moser-number.html) - Leo Moser (PDF)
 * [Analytic Geometry (1922)](http://djm.cc/library/Analytic_Geometry_Siceloff_Wentworth_Smith_edited.pdf) - Lewis Parker Siceloff, George Wentworth, David Eugene Smith (PDF)
 * [APEX Calculus](https://www.apexcalculus.com) - Gregory Hartman, Brian Heinold, Troy Siemers, and Dimplekumar Chalishajar