This book is one of two I've found that does an awesome job of synthesizing the information available in the myriad GSM speciifcations into a practical, understandable, technical description of how the system works. It explains the evolution of GSM as a set of standards and gives insight into some of the motivations and reasons things came about, instead of just drying describing them.

For myself and my development team, I have found this book to be an excellent way to really understand the technology. The 3gpp standards are of course the last line of authority, but it's very difficult to get a clear picture of how stuff actually ought to work from reading them alone.

So...read this first.