Amazon.com just posted my four star review of VPNs Illustrated by Jon C. Snader. From the review:
VPNs Illustrated is a great book for those wishing to understand network traffic at the packet level. Author Jon C. Snader was inspired by the earlier TCP/IP Illustrated volumes, and tries to reproduce the Tcpdump-style material found in Stevens' classics. The level of detail found in VPNs Illustrated easily outweighs any problems this book might suffer, so I recommend you read it for in-depth knowledge of VPN traffic.
I think this book got under-rated by two previous reviews, and slightly over-rated by one reviewer. I would definitely check it out, at least by browsing at the book store. You'll probably like it if you read this blog and do packet analysis.