The Cisco Press book Troubleshooting Cisco IP Telephony, is a gem. I spoke on troubleshooting VoIP in converged networks at VoiceCon this week (see the prior post) and some of the ideas for problems came from this book. It is easy to read and is organized in a way that also allows it to be used as a reference when you're encountering a problem and would like some troubleshooting ideas. At 973 pages, it might be intimidating, but I found it easy to use. There is a two-page quick index of major topics on the inside front cover where it is easy to find. I wish more books did that. A full index is in the end as well, if you don't find the topic you're seeking in the quick index.
The first few chapters explain IP Telephony architecture, troubleshooting methodology, and troubleshooting tools. The next chapters cover phone registration and various hardware like phones and gateways. If you're looking for ideas on troubleshooting a voip problem, you'll want to read through the chapters on voice quality, call routing, call preservation, transcoding, and conference bridges. These chapters are the core of the operation of voip and a good understanding of what can go wrong is important when you're troubleshooting. Next, you'll find information about music-on-hold, call admission control, voicemail, and survivable remote site telephony (SRST, useful when your branch connection to the central call controller goes out). Finally, there are chapters on things that aren't strictly voip, but are important to understand: applications, database replication, and LDAP integration.
I found the writing style to be very conversational, making it easy to read and understand. If you're working on voip or your organization is thinking about it, I highly recommend getting a copy.
-Terry