Hmmm, well I had to plump for 'other flavor books' in the end. I have gotten plenty of mileage out of more 'crunchy' books like the class splatbooks and similar elements, but I run a homebrew world.
So in the long run, the things I find most important are ideas for settings, locales and people. Books like Thunderhead Games'
Bluffside and S&SS'
Hollowfaust are most valuable to me. These provide me with detailed locales to set adventures, provide ideas for the adventures themselves, and give me many interesting NPCs that perhaps I would not have developed for myself.
I agree with rounser that modules are rarely of any value to me. Adapting them to fit in my setting (I have no standard demi-humans, and far fewer 'monsters') means that the conversion process is generally
far too time-consuming. By thr time I am done, I feel that I would've got further developing my own ideas, which I am at least assured
do fit with my setting.
Still almost anything, from novels to films, textbooks to documentaries can provide grist to the mill of ideas. Some of my best plots are pillaged from history books - real people are
always far weirder than anything you could dream up for yourself!
