As of 2013, this list is no longer on the Allworx portal, so I am posting it here for posterity. Most of these links are PDF. If this violates copyright, please leave a comment below and I will remove the link.
1. TCP/IP Illustrated: Volume 1 (ISBN: 0201633469)
2. SIP Demystified (2002) (ISBN: 0071373403)
3. Allworx Networking White Paper
4. Allworx VoIP White Paper
5. Telecom for Dummies (ISBN: 0-471-77085-X)
6. The Essential Guide to Telecommunications (ISBN: 0-13-148725-6)
7. Allworx Classroom Training Manual - this was in the Allworx Portal