Don't know about a web site but I've seen the book at Barnes and Noble. Don't know the name but it's essentially a blue book for instruments. It was lumped in with all the other guitar instruction and music books.
In my experience the money a guitar is worth depends very little on what the blue book says. Believe it or not, eBay seems to be a very good indicator of what something is worth, or at least what the majority of people are willing to pay.
What are you selling or buying?
YMMV