Add The Art of Prolog (#3066)

free-programming-books.md

@@ -2050,6 +2050,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Prolog Problems](https://sites.google.com/site/prologsite/prolog-problems) - Werner Hett
 * [Prolog Techniques](http://bookboon.com/en/prolog-techniques-applications-of-prolog-ebook) - Attila Csenki, Bookboon. (email address *requested*, not required)
 * [Prolog Tutorial](http://www.cpp.edu/~jrfisher/www/prolog_tutorial/contents.html)
+* [The Art of Prolog, Second Edition](https://mitpress.mit.edu/books/art-prolog-second-edition) -  Leon S. Sterling, Ehud Y. Shapiro (Open Access)
 * [The First 10 Prolog Programming Contests](https://dtai.cs.kuleuven.be/ppcbook/) - Bart Demoen, Phuong-Lan Nguyen, Tom Schrijvers, Remko Tronçon
 * [Warren's Abstract Machine: A Tutorial Reconstruction](http://wambook.sourceforge.net) - Hassan A¨it-Kaci