I use a Palm Tungsten II. It does exactly what I want it to do, it's very small and easily prop-up-able against any cats that want to share my pillow, and it's backlit so I can read at night in bed.

I prefer .html and .txt files that I convert into files that I can read with iSilo (I can look up what format that is once I get home). I'm not sure I'd have a need for an ebook library, but thought I'd wheigh in on the "what are you using" question.