I use and like:
The Blue Book of Gun Inventory,
https://store.bluebookinc.com/Firearms/ISP.aspx.
You can slide through the listing that are arranged alphabetically and pick your gun out of the Blue Book on line and then click, add it to my inventory. It allows pictures and condition and a lot of useful information.
These are the same people that produce and publish the identification and serialization book that we all refer to so much.