It covers the three of the main methods you’ll probably use for most ebook transferring to iBooks — dragging and dropping through iTunes; downloading books through iBookstore; and opening up PDF books through email.
For converting those ebook files into EPUB form, Calibre (free) is a must-have. It’s pretty much the best and perhaps only ebook conversion software you’ll need for 99% of getting your ebooks to and from one device to another.
The Ebook Reader has more details on EPUB and iBooks and another explanation of importing those files via iTunes —
For the audio and visual types: a video walkthrough for getting EPUB ebook files into iTunes and customizing your book cover art. CNET: “How to load EPUBs into iBooks”
If you’re trying to publish an EPUB book to the iTunes store, you might want to check out this anecdote, courtesy of ZDNET, “Six tips for getting Apple’s iBooks to accept your ePub file“