I've actually read a couple of books (short ones!) on my Nokia 3650, but it's only good for as-I-wait-for-the-doctor sort of desperate reading, not for real leisure (imo, of course, to each his own). I read somewhere that the length/size of a typical book (A5ish) is actually the perfect size for reading comfort and comprehension, both across and down. I tend to agree, larger pages feel long-winded, and shorter ones keep giving me mental "jumps/starts"; I am the type of reader that gets totally lost in books (to an unbelievable extent, lol), but magazines or booklets/Nokia I never get into so deeply. That's something from my reading experience that I'm not willing to give up.
As you say, battery life doesn't seem too significant at the numbers these readers are handling, hehe, but it's a good point all the same.