iBooks Author is a free app available in the App Store on your Mac that you can use to create and publish ebooks. Using the tools in the app, you can add artwork and metadata to a book. You can also open .ePub files that you created in a different program to iBooks Author, and then sell them on the iBooks Store or distribute them for free.
Honestly I know nothing about Mac so I did not go any further. Maybe someone with a mac will put their book on here and tell us how it went.