We've talked about publishing via RSS and via OPDS Catalogs but that is all about publishing unencumbered EPUB documents without payment to the author. While that may be your intent, there are other options. It is also possible to market an ePUb document and be compensated for that work. One attractive option is to use Apple's iBookstore which uses the agency model: you set the price and get 60% of all sales with Apple getting the remainder.
There are two basic approaches to using the iBookstore, doing all the work yourself or getting some help from an approved aggregator.
Your first step should be to acquaint yourself with the rules. They are simple and few in number. Go to iTunes Connect where you will see this :
... be sure to use the drop-down menu to choose "Books."
Here you'll see the requirements for submitting an eBook to the iBookstore.
The next two screenshots show the application and the info requested prior to uploading your .epub files. Of course it is essential that your book be exactly as you intend it to be. Involving an editor or at least other literate people as critical readers would be a wise move before uploading.
If this seems too much to bear, you always have that second option which is to involve an Apple-approved aggregator. For a small fee, these folks will handle most of what needs to be done. However, they generally do not support or market the book. That's your job and it can be a mighty challenge. There are lots of comments by other authors on this point so it's good to make an effort to find, read and consider the experience of others.
You'll want to investigate the requirements of each aggregator as they are not all the same. Smashbooks, for example, has a Style Guide that you should consult well ahead of time. Their style guide is an .epub file and they only accept manuscripts in the form of a Microsoft Word document. Smashwords also offers the option to publish via other venues in addition to the iBookstore.