I did enjoy some of the free microsoft reader ebooks that were available on their website (as well as my installation CD-ROM for my ppc).
Can't beat classics like:
Little Women and
Peter Pan
But seeing the forum topic is Ebook ideas -
There is always Isilo for converting your favourite online html books into pda format. For the Australian medico's out there, both the entire Therapeutic Guidelines and Murtagh General Practice series are available in html format for desktop and can easily be converted within minutes to isilo form. These are the two "ebooks" I use the most often.
I've always wondered why these publishers don't take advantage of the isilo format and actually distribute their reference textbooks as an ebook using isilo. (Almost all Australian medico's would recognize the two popular references). isilo format is not only easy to use, very quick to load up, utilizes a very small footprint and allows beautifully formatted pages that allows for html links allowing easy navigation

Plus, the format is universal for both palm OS and PPC.
As to the legality - my institution has access to the online html / CD-ROM version of these texts. The process of converting the to ebook form is akin to saving the webpages into your internet explorer cache (which i used to do before isilo) and then compiling it into one file - except isilox automates the process. What do other people think?
There seems to be very little discussion on medical ebooks in this forum :P