Asterisk: The Future of Telephony - Free Book Download
Or, as they put it more eloquently on that page:
O'Reilly Media's latest book, Asterisk: The Future of Telephony, written by Jim Van Meggelen, Jared Smith, and Leif Madsen is the most complete book on the Asterisk PBX system to date. The new book, announced at Astricon 2005 covers many of the new features of Asterisk 1.2. In the spirit of open source, O'Reilly has licensed the book under the creative commons license making it free to download and distribute. If you are looking for the ultimate Asterisk book, you can now download the entire book as a PDF file!
However, 3 years later (2009), that link 404s.
Regrettably, archive.org hasn't retained the zip file, but perhaps you can find it elsewhere. Filename is AsteriskTFOT.zip.