From http://support.apple.com/kb/HT4059:
"Can I copy my books onto other computers or devices?
Books downloaded from the iBookstore can be placed on up to five computers you own that you’ve authorized with your iTunes Store account. You can sync your books to all iPads, iPhones, and iPod touches you own.1 Audiobooks, PDF files, and ePub files you've added to iTunes will appear in Books under Library. To sync Books to your device, connect it to your computer using the cable it came with. In iTunes, select your device then click the Books tab. Choose the books you would like to read on your device then press Sync. Books will sync to iPad, iPhone, and iPod touch even if iBooks is not installed; to read synced books, download iBooks from the App Store."
"1Requires iPhone and iPod touch with iOS 4 or later. iBooks is not compatible with original iPhone or iPod touch (1st generation)."