TL;DR version: go big or self-publish.
(The following was written to help me think through the possibilities for a couple of project I’m involved with. It may or may not be useful to others. Also, none of the following takes the need to diversify into consideration which could completely change the picture. As always, YMMV. And ‘book’ for the purposes of this blog post is any project, digital or print, that is primarily intended to be read.)
If you’ve written a book, there are basically five things you can do with it.