I love WFRP 3e, but I'm new to Warhammer as a whole... is there a book out there from the older editions that has a world gazeteer or anything of the sort? I've used the prewritten adventures, and I'm finding I'm not familiar enough with the rest of the setting to start writing my own yet.

If you want a gazetteer and some Empire location source then I'd suggest the easiest way to get it is to pick up the 2nd ed sourcebook Sigmar's Heirs. If you just want some ideas for writing typical WFRP type scenarios then maybe Plundered Vaults might be a better bet.

The first ed Shadows over Bogenhafen (the versions combined with Mistaken Identity) has some excellent and seminal source/gazetteers and adventure, but it's difficult to find, and expensive these days.