Whether you have a passing curiosity about Asterisk or your livelihood depends upon it, this is THE Asterisk book you need on your shelf (or e-reader).
Originally called "Asterisk, The Future of Telephony", the new title is more apropos. Not only is "the future of telephony" a present-day reality, but all earlier editions have been the de-facto definitive guides as well. The third edition welcomes contributions from brilliant mind of Russell Bryant.
Our venture capital fund uses Asterisk to bring competitive advantages to our our portfolio companies. I use Asterisk personally to build "crazy powerful" and fun telephony solutions for myself and family. With the stunning power and versatility of Asterisk, it's no wonder there are more Asterisk servers running than all the Google and Facebook servers in the world!