You can find many SciFi books for PPC in several different formats. I buy mine from
www.fictionwise.com . I'm not so sure about medical ones.
Your PPC will have Microsoft Reader already installed in the ROM (although you will need to download an update from MS) so Microsoft's .lit format might be a good format to start with.
For books in Adobe format there is a version of Acrobat Reader for the PPC. I haven't used it, so can't comment on how it shapes up to the other formats.
Check out the threads here, there is a lot of discussion on the attributes of the different eBook formats and the readers for them.