I don't know if it's the best option nowadays but it used to be. You can publish your book online and they'll supposedly automatically give it an ISBN and send it to places like Amazon - but I last read all about how it worked some years ago so it could be different now. The good thing was that you used to be able to choose from a lot of different book sizes and paper qualities so you could get your book to be pretty cheap for anyone who'd want to buy it. (And, of course, you could make it an ebook.)

I never had a book on there that cost any money, but I did put up some free "magazines" I made and people did download them without me having advertised them.