I've always found the "Blue Book" kind of a waste of time. As stated before, the prices seem to tend to be "low ball" and are good only if being used for buying. The problem is if you are the one that is buying, you usually can't find anyone willing to sell at those prices. (Unless they don't know any better.) It's not useful for identification due to the lack of pictures and minimal descriptions.
I recently picked up a copy of the "Standard Catalog of Firearms" and although the pricing is a little shaky in some areas, it's a great reference with over 1,700 photos and lots of info. The main problem is its lack of military firearms. There is a separate volume "Standard Catalog of Military Firearms" I'm going to need to get.
Other good choice’s are Flaydermans (Only good for prewar and antique US guns) and the Standard Catalog of Smith & Wesson (If your bent is S&W. Also, with the SCSW, you have to be very critical when judging condition to realize the higher values listed.).
On-line auctions (Gun Broker, et al) and sales sites (Guns America, etc.) can be useful. The main thing I don’t like about them is that, for the most part, they show what some folks would like to get for their guns, not what they actually are selling for.
I’ve always figured that the actual value is somewhat less than what you see being asked. This is mostly true on GA. On GB, as long as the gun doesn’t have a Reserve, you have to watch the auction actually “close” to get a feeling for the selling price.
As far as I’ve been able to figure out there’s no way to look at “closed” auctions on GB to see actual selling prices. (Maybe someone can let me know if there is a way to look at closed auctions.)