The Blue Book of Gun Values is the definitive source of gun values. It is immensely useful if you are buying or selling used firearms.
If you do not do a lot of buying or selling, and do not want to purchase a US$30 book every year, then the Blue Book of Gun Values online database may be very useful.
A lot of the Blue book content is available for free. The modern/popular manufacturer, such as Marlin and Ruger, information costs $10.
The database is available here.