Add App Inventor category and book (#5360)

* Add App Inventor category and book

* Fix header level

* Add additional App Inventor book

check_urls=free-programming-books.md

* Switch link to official page for added book

check_urls=free-programming-books.md

books/free-programming-books.md

@@ -6,6 +6,7 @@
 * [Alef](#alef)
 * [Android](#android)
 * [APL](#apl)
+* [App Inventor](#app-inventor)
 * [Arduino](#arduino)
 * [ASP.NET](#aspnet)
 * [ASP.NET Core](#aspnet-core)
@@ -998,6 +999,12 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Mastering Dyalog APL](http://www.dyalog.com/mastering-dyalog-apl.htm) (PDF)
 
 
+### App Inventor
+
+* [Absolute App Inventor 2](https://amerkashi.wordpress.com/2015/02/16/absolute-app-inventor-2-book/) - Hossein Amerkashi
+* [App Inventor 2](http://www.appinventor.org/book2) - David Wolber, Hal Abelson, Ellen Spertus, Liz Looney
+
+
 ### Arduino
 
 * [Arduino Programming Notebook](http://www.lulu.com/shop/brian-evans/arduino-programming-notebook/ebook/product-18598708.html) - Brian Evans