Not first-hand experience, but a friend of mine published a book online and got taken for a ride. The general caveats for all book publishing apply, but doubly so on the internet: watch out for people who want you to pay for something, point you towards specific services that want you to pay for something, and don't expect much. My impression of most online publishing is that it amounts to vanity press.
I know you can sell directly through Amazon and their Kindle platform, and they take some fairly substantial percentage of the price. I've read 55% various places, but again I've got no first-hand experience. I know in the past there was a registration fee associated with setting up a seller's account, but I'm not sure what the deal with that is anymore. There are lots of how-tos online for selling your book as an ebook, that's probably a good place to start. Just make sure it's up-to-date, since a lot of stuff has changed with their policies in the last couple years.
Anyway, good luck to you. I've got endless respect for people who actually manage to finish something book-length, and even more for people who run the gauntlet of publication.
