Ted Wallingford sent out a note this weekend to alert folks to the publication of his new book "VoIP Hacks"., given Ted's background I'm sure this will be a worthwhile read.
The VoIP security book space is getting crowded these days. In addition to Ted's book there's the upcoming "Hacking Exposed VoIP" by Mark Collier and Dave Endler, and the two sitting on my nightstand: "Understanding VoIP Security" by Alan Johnston and Dave Piscotello, and "Practical VoIP Security" by Thomas Porter and Jan Kanclirz.
Given the amount of VoIP security FUD, and the reputations of all these authors, I'm confident that all of these books do a great job explaining the real threats, and how they can be mitigated. Now if I could just find the time to read them. :-)