Hehehe, I know you like books :). Other nice books about FreeSWITCH are also published by Packt, like this one. Their title are: "FreeSWITCH 1.6 CookBook" and "Mastering FreeSWITCH". We know the authors, we can guarantee for them.

Another book I always counsel is "VoIP Deployment for Dummies", published by Wiley. It's not for dummies at all, and (even though it was published in 2009) will give you a complete general course 400 pages long (covering topics including SIP, LAN metrics, dealing with ITSPs, and many more).

As reference works on SIP, RFCs are there for free, and the book from Johnston, "SIP, understanding the Session Initiation Protocol", Artech House, I found it complete and easy to consult when I look for details. For the architecture and the working of SIP based complex systems, "Session Initiation Protocol, Controlling Convergent Network", McGraw, is a good one.

I want to definitely signal here the book of my friends at OpenSIPS project, Bogdan-Abdrei Iancu and Flavio Goncalves, "Building Telephony Systems with OpenSIPS", again Packt publishing, because in its first part it lays down so well a working knowledge of SIP (that's needed for reading the rest of the book). Knowledge that's rooted in real implementation experience. Also, OpenSIPS as a SIP load balancer, proxy, registrar, etc is a project complementary to FreeSWITCH, and they are very often used together.