Now, at this point, its common knowledge that brick and mortar publishers are suffering, and that e-book readers are becoming increasingly popular, so it’s easy, and honestly probably correct to assume that E-books are going to replace the old paper books that we are all so used to.
Personally I like this trend, because it lowers the barrier to mass distribution massively, but don’t assume that it’s completely mainstream yet. It’s getting there, but currently it seems like paper books are still outselling e-books, overall (Amazon notwithstanding).
Also, e-books have some controversy, in the form of how Libraries have to deal with them. E-book publishers want to make libraries rebuy the books after a certain amount of times being checked out. Unsurprisingly, libraries are a little pissed about this.
Anyway, I’m meandering off the topic. Generally, if you want your e-book to be published, you’ll need to find an e-book publisher (do your research, and make sure they’re reliable). Also, you’ll need an ISBN, if you want e-book publishers like Amazon or Barnes & Noble to pick it up. You can get one for your book from here for about $30.