The two fattest books I can think of, which are Tolstoy's War and Peace, and A Suitable Boy by Vikram Seth. They both weigh in at around 1500 pages, and last time I read War and Peace, my hardcover just about broke my wrists. (At least The Lord of the Rings is available in three volumes.)
I want to re-read both, and figure that if I am going to try out this e-book thing, I might as well give the iPad a serious work-out. If it can cope with those two, it can cope with anything.