O'Reilly's Virtual Private Networks by Charlie Scott, Paul Wolfe, and Mike Erwin.
My review of this book can best be summed up in two words: Don't bother. The printing date on this book is January 1999 and I don't know that any further updates have been done. Generally speaking, the information in this book is outdated and probably no longer relevant to most people interested in VPNs in 2005.
The book covers remote access concepts and network design, PPTP, L2TP, PPP over SSH, Cisco firewalls, and something called Alta Vista Tunnel that I've never heard of, and I don't think it exists any longer. My book also had two copies of chapter four.
PPTP and L2TP are still active VPN technologies, and they're included with OS X server, but most VPNs these days are IPSec, and that topic isn't covered in the book. I don't know that PPP over SSH is any more common. And the book's information about Cisco firewalls covers commands that are now deprecated in favor of access lists.
This book was probably a good reference in it's time, but that time has passed. I can't recommend it to anyone and I'd suggest that people interested in VPNs look elsewhere for a more modern book.