Many of us accumulate a bunch of these along the way. I was reflecting today on those I seem to consult over and over, over time. These are my favorite four "go-to" soft-bound documents. They may not answer all my questions, but I almost always pull something useful from the four towards whatever. Here are mine:

Buxton's Guide to Foreign Firearms
Stoeger's 1939 Shooters Bible
Gun Digest 15th Year Treasury
Stoeger's 1st Shooter's Bible Treasury

Buxton often gives me at least an illustration and short rap on a commercial, foreign made shotgun or rifle which I just couldn't come up with quickly elsewhere. The 1939 Stoeger's Jubilee Edition Bible illustrates the creme de creme of the American pre WW2 guns that are perennially my favorites. The Gun Digest 15th Treasury is consulted over and over for the Baron's articles on European proof marks (plus a timeless Nash Buckingham piece), and the 1st Stoeger's Treasury provides a broad-span look at primarily American guns compiled mostly from the post WW1-pre WW2 period.

I'm always interested in adding more. I'd be interested in hearing what soft bound documents are your "go-to" ones.