I've only read epub books and pdfs on the 600 but both have been fine - generally the formatting has been excellent on the epub versions and not-so-good-but-still-readable on the pdf files.
I'm not entirely sure that you should convert library books as I think they are protected? Well at least the epub files from my library are.
After the library period has expired, the books will no longer be viewable. I think you can borrow them again from the library though.