Anthologize is a free, open source plugin for WordPress. It makes it quite easy to organize an ebook from a subset of content on your WordPress site. It has a few bugs and quirks that need to be worked around but is otherwise quite good. I have a tutorial pending publication at Tuts+ which will describe hot to use Anthologize to publish your own ebook.
MagPress is a $49 plugin which may provide a more robust experience than Anthologize. I haven’t yet tried it.
PressBooks seems to be a way to use it’s WordPress system to compose a book, but it doesn’t seem well-oriented towards publishing from your existing site.
Blurb advertises itself as a complete book publishing platform. While it used to have a way to convert WordPress.com (not self-hosted blogs) to books, I can’t seem to find that capability any more. It still has solutions for other blogging platforms, but it’s usefulness for WordPress publishers is unclear right now.
You can download the BookPress plugin but their website is now defunct.