Added A Primer on SQL, Second Edition.

free-programming-books.md

@@ -1573,6 +1573,7 @@ See also [TeX](#tex)
 
 
 ###SQL (implementation agnostic)
+* [A Primer on SQL, Second Edition](https://leanpub.com/aprimeronsql)
 * [Developing Time-Oriented Database Applications in SQL](http://www.cs.arizona.edu/people/rts/publications.html)
 * [Learn SQL The Hard Way](http://sql.learncodethehardway.org/)
 * [SQL For Web Nerds](http://philip.greenspun.com/sql/)