I heard about...

manybooks.net

...on the Security Now podcast (of all places!). The site provides free ebooks in many formats (including one for the Apple Newton). A web page specially formatted for the Sony PSP is also available: psp.manybooks.net. I just downloaded Rebels of the Red Planet in the Special PDF format. So far, it looks like it was formatted for a PDA or small screen ebook reader. More later...