It appears that there is a new book out on VoIP security named, rather simply, “Voice over IP Security“. It’s from Cisco Press and written by a Patrick Park. I haven’t seen the book yet but ITworld has an interview with the author. Amazon.com of course has some user reviews as well.
Good to see additional books coming out into the field. It will be interesting to see how this compares to the others out there.
P.S. If you have the book and would be interested in writing a review for this site, please feel free to contact me.