Joe Moell is the author of Transmitter Hunting -- Radio Direction Finding Simplified, and the regular "Homing In" column in CQ VHF magazine. This article gives a great introduction to the topic. The rest of his site also includes lots of info and the latest news from the t-hunting community.
A big collection of easy and fun projects for the do-it-yourself T-hunter. Includes plans for the 3-element "tape measure" Yagi antenna and active attenuator shown at the Mike and Key swap meet in March 2005.
Though focused on hunting in the San Francisco Bay area, this site gives you an idea of what you can expect on a typical "mobile" (car-based) hunt. Also includes info on T-hunting basics and equipment, as well as tips and tactics.