Can I open eBooks bought on the iBook store on a Kindle?
?iBooks uses books published in the ePub format
?To add ePub files from outside the iBookstore to iBooks, they must be DRM-free and synced to your device using iTunes
iTunes can show you the kind of book you have by selecting the book and then choosing File > Get Info. If the Kind is marked as Book, this is a book you downloaded from the Internet. If it is listed as Purchased Book, it is a DRM-free book downloaded from the iBookstore. If it is listed as Protected Book, it is a copy-protected book from the iBookstore.
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.
Kindle books can only be viewed and read in Amazon's Kindle app for the iPad
you can try calibre, It'll convert between a number of formats
Stanza for the PC can also be used to convert a number of formats to the Kindle format via export. They can then be loaded onto the Kindle thru the USB port.