I went to the library a couple of weeks ago and was surprised to find how very few books were in the library system that actually discussed cults. I did find two though and have been working my way through them. The internet has been another useful tool as well.
In the past, I did just enough research to be able to throw terms around with some accuracy. For the Witness Tree I read a book on local history and researched the Bald Knobbers. Frankly, I feel like I didn't do the group justice and someday I hope to rectify that, in a sci-fi/conspiracy sort of way (This story really captured my imagination). I also paid very, very close attention to the atmosphere in the small town I based the story on and I drew on local people I met. The finished product wasn't great, but writing that one taught me a lot about authenticity.
For Troubled_Waters,in order to get a good feel for what makes bad guys bad, I read a book written by a profiler who'd tracked down serial killers and psychopaths. But my readers saw so little of the villain, that I'm fairly sure the research wasn't really put to use. I researched linguistics specialists and the FBI in enough depth to know what offices were there (and then totally disregarded the facts with respect to the Jefferson City satellite office, which is actually very, very small and not at all how I painted it up to be). I learned what they called each other (Special Agents in Charge--SAC, Deputy Director, etc) and a bit about their cybercrimes division. I've been to Jefferson City exactly three times (taking mental notes every time), so I've had to lean on the help of a local for specifics. The only thing I might have done better there is include the names of local restaurants here and there and I may rectify that before actually sending a full-copy to an agent. And locals will know that Internet Cafe's are not common there at all. I make no apologies for the liberties I took, I'm just sayin' I know I did it.
This time, because my characters will be spending a fair amount of time in the cult environment and in the presence of the bad guy him/her (?) self, I want to be sure the atmosphere is authentic. I do think I'll probably move the cult to an area with geography I'm more familiar with so that I'm not guessing quite so often. This time I will lean on landmarks on the names of towns as I know them.
I'm obviously not a big embracer of the "Write (only) what you know" philosophy (That would make for a boring--and very short--book). I am of the "write what you can write about convincincingly" camp. However, someday I'd like to have the time and money to do the kind of research Christopher Moore did for Fluke or, I Know Why the Winged Whale Sings. He actually spent a fair amount of time in the presence of marine biologists as they studied whales and dolphins--and I think this is way cool, by the way. Maybe a publisher will pay me to go to someplace tropical for a year so I can emerse myself in the local bea--I mean--culture and write a book inspired by the experience. And, no, the tiny pink umbrella stuck in my pencil holder has nothing to do with this idea