I received a message on twitter asking if I knew how to test if a GSM Sim900 module was working.  I have not worked with that module hands on but did recall a great write up on it from a trusted source, John Boxall of http://tronixstuff.com. I learned of John through Arduinos site under Mannuals and Curriculum. His book, Arduino Workshop, was listed right along with Massimo Banzi's Getting Started With Arduino.

What impressed me most with this book is that all 65 of the projects from the book can be found on the authors website along with videos documenting his build of the project.

http://tronixstuff.com/.  Actually, each of the projects from the book are well documented on the site and has great educational content on it's own. 

In the spirit of Open Source John publishes all his stuff under "Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License." allowing you to copy and redistribute the material in any medium or format long as you give credit to the original source and are not using for commercial purposes.

If you find the projects on Johns Site useful please consider buying his book.