Check out this useful post from the How to eBook blog with help on deciding what format in which to publish your book
Which format should authors publish their books in – and why?
Many authors will simultaneously release a book in multiple formats. At the very least, they will have a print and digital edition published at the same time. Others will do an audiobook, perhaps a little later on. Some will publish hardcover and trade paper at the same time while some may only do a mass market paperback edition. There are also publishers who release a print-on-demand version while selling the digital and audio book editions.
What makes sense for you?