For other online booksellers, you will have to consult their own web pages and forums about what formats they require. These forums are only for Apple's iBookstore.
The iBookstore accepts .ibooks and .epub formats only. The former can only be created using Apple's iBooks Author app. The Apple app for .epub format is Pages, but there are many others you can use.
PDF's are not really designed for editing. I suggest you use google to find some online conversion service and convert your book to .doc and edit it in that format. Then you can use other services to convert .doc to .epub, or you can open it with Pages and export to .epub or you can import .doc into iBooks Author and export as .ibooks.