I don't do it often, but I have read books on my PDA. When I used a Palm, I read Sun Tzu's Art of War. On my spiffy new iPAQ, I've got a copy of Adam Smith's Wealth of Nations that I'm trying to wade through. I've also got a dictionary, Bible, and the SRD on it, for reference.
The print is slightly smaller than most books, but not too bad (and most things have a zoom setting). I can read in bed without my contacts in.
I'd probably read more books on my PDA, but DRM means that I'd still have to pay something of note for the book and am then subject to some headaches. About the only thing I read are "classics" that have entered the public domain and are, thus, free. They are also incredibly dry, most of the time.
If DRM doesn't bother you, though, there is a reasonable selection of material.