Anthologize is a free, open-source, WordPress-based platform for publishing. Grab posts from your WordPress blog, pull in feeds from external sites, or create new content directly in Anthologize. Then outline, order, and edit your work, crafting it into a coherent volume for export in several formats, including PDF, EPUB, and TEI.
I agree with an earlier issue request -- the ability to customize a book's feel, or "theme", would be an extremely valuable addition to this plugin.
Changing typeface, the option for adding chapter artwork, etc, etc. As also noted, Pressbooks does a good job at this.