ePub, of course, is a supported format. Other supported formats are:
- ePub with DRM
- iBooks (Generated by iBooks Author)
The iBooks app itself supports the same formats (from the iBooks FAQ):
iBooks supports the industry-leading ePub digital book file type. You can sync freely available ePub files to iBooks using iTunes, or purchase or download books from the iBookstore. iBooks also supports PDFs, which can be manually synced to iBooks. In addition to ePub and PDFs, iBooks 2.0 and later also provides support for the new iBooks format used in iBooks Author. Other digital book file types are not supported at this time.