Are there official publishers who work with Esperanto authors? Or is there some sort of semi-vanity publishing arrangement for most Esperanto works?
One of the more active (though somewhat local) publishers is Espero - http://www.espero.sk
If you were looking at just publishing e-books, there are many online (non-Esperanto-specific) services you can create and sell your books through.
Some of these are:
It's mostly technical books that get sold through these platforms, but they do support non-technical books in any language. Leanpub specifically has Esperanto as a language option you can select when creating a book.
One good site to publish ebooks in Esperanto is Smashwords. They will upload the book to many other retailers on your behalf. Sadly they can’t be sold in the Kindle store because Amazon currently rejects books in Esperanto.