Personally, I rely on the prices being advertised in Gun List. It's a more realistic representation of what the market will bear than the books, IMHO. I've never seen guns selling for what the blue books say they're worth.
I then cruise the gun shows and shops; you get an idea of what dealers are paying (from Gun List) and what dealers are asking (shows, shops), and figure the value somewhere in between.
Just my $.02 - keep the change.