Adding sass book(pdf) (#6743)

* Added sass book(pdf)

* Arranging alphabetically

* Rectifying blank lines between last section and next heading

* changing link to be direct to the pdf for dowloading

books/free-programming-books-langs.md

@@ -948,6 +948,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Learn CSS Layout](http://learnlayout.com)
 * [Learn CSS Layout the pedantic way](http://book.mixu.net/css/)
 * [Learn to Code HTML & CSS](https://learn.shayhowe.com) - Shay Howe
+* [Learning sass](https://riptutorial.com/Download/sass.pdf) - Compiled from Stack Overflow documentation (PDF)
 * [Magic of CSS](https://adamschwartz.co/magic-of-css/) - Adam Schwartz (HTML) (:construction: *in process*)
 * [MaintainableCSS](http://maintainablecss.com)
 * [Pocket Guide to Writing SVG](https://svgpocketguide.com) - Joni Trythall