The books give you an indication / but prices are flucuating too much the last couple of years to rely on any of them.
I would do an internet search / maybe look at gunsamerica.com / and talk to some knowledgeable dealers in your area that deal in used guns ....and you should get a better more useable range for price.
If you're pricing standard production guns / like Glocks, H&K, Sig, Browning shotguns, etc ..... your local stores are a better idea than anything. Talk to some of the shooters are your local ranges / they always know who has the better overall deals on guns new and used in the area .....
I'm a gun shop junkie ....always cruising my favorite shops - mostly for used guns / but even in my area there are 3 or 4 shops, I rarely visit .... same old junk there, prices are not good, no reason to go ....