My wife is a managing editor (and used to be a hands-on-board) editor.

Books are edited.

There's a great book in the archives of Joel on Software (maybe he's even had it done); possibly two or N -- but that requires filtering. And then combing and rewriting/patching not to depend on links anymore (so it makes sense like five years from now, dig?), nor on an ongoing blog narrative. Then understanding the major themes to consolidate multiple blog posts in chapters.

I don't know that this has been done with a blog, but yes from lecture notes and slides, etc. Usually the writer has to come up with text (or hire a ghost writer); most of the time, he edits his work to some extent. But manuscripts never arrive ready for print.

You can see this in Lulu "books". I own Nobel Prize-winner Vernon Smith's autobiography published by a vanity press and holy hell what a mess.