The Grand Duchy of Karameikos
Lowell Francis has a review of one of my favorite D&D supplements, The Grand Duchy of Karameikos on his Age of Ravens blog. This D&D Gazetteer set the tone of the entire series. I used it in several campaigns, even dropping in with only a few changes into Arn a time or two. You can read Lowell’s comments on The Grand Duchy of Karameikos here. Written for BECMI/RC, it could easily be used with Microlite74 or any TSR era version of D&D (or retroclone). While it isn’t cheap if you want a great copy, reading/playing copies are relatively easy to find in my experience, something that cannot always be said of later gazetteers.