Merge pull request #240 from d3noob/patch-1

Update free-programming-books.md

free-programming-books.md

@@ -598,6 +598,7 @@
 ####D3.js
 
 * [Interactive Data Visualization for the Web](http://chimera.labs.oreilly.com/books/1230000000345/index.html)
+* [D3 Tips and Tricks](https://leanpub.com/D3-Tips-and-Tricks)
 
 ####jQuery