Adding in Calculus books from OpenStax (#7743)

* Adding in Calculus books

* Added Calculus Volume 2 book from OpenStax

* Adding 3rd Volume of Calculus textbook from OpenStax

books/free-programming-books-subjects.md

@@ -430,6 +430,9 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Calculus I](https://resolver.caltech.edu/CaltechBOOK:1985.001) - Jerrold E. Marsden, Alan Weinstein
 * [Calculus in Context](http://www.math.smith.edu/~callahan/intromine.html) - James Callahan
 * [Calculus Made Easy](http://www.gutenberg.org/ebooks/33283) - Silvanus P. Thompson (PDF)
+* [Calculus Volume 1](https://openstax.org/details/books/calculus-volume-1) - Edwin Herman, Gilbert Strang (PDF)
+* [Calculus Volume 2](https://openstax.org/details/books/calculus-volume-2) - Edwin Herman, Gilbert Strang (PDF)
+* [Calculus Volume 3](https://openstax.org/details/books/calculus-volume-3) - Edwin Herman, Gilbert Strang (PDF)
 * [Category Theory for the Sciences](https://mitpress.ublish.com/ereader/77/?preview#page/Cover) - David I. Spivak
 * [CK-12 Probability and Statistics - Advanced](http://www.ck12.org/book/Probability-and-Statistics---Advanced-%2528Second-Edition%2529/)
 * [CLP-1 Differential Calculus](https://www.math.ubc.ca/~CLP/CLP1/) - Joel Feldman, Andrew Rechnitzer, Elyse Yeager