I'm a big fan of Asterisk, the VoIP exchange system, even if I'm not yet in a position to make use of it at the moment.

Asterisk is difficult to configure and is such a rich and capable application that getting to know how to configure everything can be a problem, so a good book is always useful.

O'Reilly's Asterisk book was released under a creative commons license, and it is now available to download for free, as noted here.