iBooks supports ePub digital book file type. You can sync freely available ePub files to iBooks using iTunes, or purchase or download books from the iBookstore.
Publishing ebooks at iBookstore:
Apple has established currency-specific pricing tiers for all eBooks sold in the iBookstore in the U.S. and international regions.
Apple takes a 30% commission on all sales.
How much do you make as an author per sale?
Apple retains 30% of all revenue from sales on the iBookstore.
The author receives the remaining revenue.
For instance: a book price $9.99 ebook on apple (on iBookstore) will give you $6.99.
For epub projects (on iBookstore) pricing must comply with Apple's guidelines.
More info at their website: http://support.apple.com/kb/HT4059iBookstore