I think you mean for e-readers, right? Publishing for Kindle on Amazon is easy if your book is mainly text-based. Go to Kindle Direct Publishing and you'll find guidelines for formatting. Essentialy, they can take a word document and convert it. If you have charts, images, etc. It gets a little difficult.
I also published with liberwriter. They have software that you can use for a fee and they are VERY helpful in that they preview your document and make suggestions. I think I paid $25 and did it myself. The reason I chose them to format my second book for Kindle is because I had pictures embedded with the text and wanted to be sure it was formatted correctly.
There's smashbooks.com and lulu.com and various other services for various devices. If you're trying to get your book into the Apple store, I'd go with bookbaby.com because it's difficult to get into Apple by yourself.