OIne creditable source is "The Blue Book of Gun Values". It lists MOST of the guns that have been produced over the years, with a description of how to judge condition, and values for the various conditions. It also has in the index in the back of the book serialization codes for many of the manufacturers, how to interpret them in regards to date of manufacture. Most larger bookstores should carry this reference, and possibly some of the larger firearms dealers.
I'm sure others on here may give you some other leads....