I've been looking at Scribd, a new Web2.0 company described as "Youtube for documents".
They are going to have serious copyright issues if the few docs I noticed there are any indication. For example here's one book that I saw posted in full text:
and there are plenty more like this. Somebody obviously went through the laborious effort of scanning and then OCRing every page. There is no formatting, just page after page of sentences.
One neat thing they offer is an MP3 version of any document. They use Nuance TTS in order to create a computer-generated podcast of any document. I'm sure it's absolutely horrible to listen to a long work this way, but better than nothing when you're in a pinch.
Meanwhile, if you really are serious about online books, note that you can get free access to zillions of technical books at Safari.com by going through the Sunnyvale library home page.
You don't need a library card or any registration whatsoever.
Virtually all the O'Reilly books, plus books from MS-Press, Peachpit, and many many more are on line.