Not an easy thing to do without knowing what kind of eq your going to run everything on. One of the most general books that I could find on IPSec, encryption and VPN was the Cisco Press book - Managing Cisco Network Security.
It's not a specificas it should be in regards to the PIX - but it does provide a ton of information about IPSec and VPNs and how they work - preshared keys - CA's etc...
I'm sure that there are others out there - but I have not read any of them.
One other that Ihave that has helped - another Cisco book - is Cisco Secure Internet Security Solutions. Another good informational book.
But if you implementing a different VPN/firewall etc... solution than Cisco - I would look for books on those products. The general books tend to be a bit light product wise. Building internet firewalls by O'Reilly is a really good "general" firewall book - but don't expect to read it and go and setup a PIX or Checkpoint firewall. And others like it that I have read took a lot of time that I didn't need to spend because the stuff is covered again in the book that you buy to setup the specific equipment.
Good luck,
Duke