Asterisk : the future of telephony, 1st ed. / Jim Van Meggelen, O’Reilly, 2005
This book is about installing and configuring Asterisk, the open source phone system. It covers voice mail, call conferencing, interactive voice response and other services. The Open Book (2nd ed) is online at O’Reilly (one large pdf).
Asterisk for dummies / by Stephen P. Olejniczak and Brad Kirby, Wiley, 2007