Virtually every Emacs geek knows about and has probably purchased Mickey’s Mastering Emacs. Álvaro Ramírez wanted a bit more: he wanted to be able to search it from Emacs.
That turns out to be surprisingly easy. Just download the epub and convert it to Org mode, and you’re all set. There’s a little cleanup involved but it seems a reasonably easy process. Check out Ramírez’s post for the details. If you want to be able to bring up Mickey’s wisdom as you’re working, this seems like just the thing.
UPDATE: Matering → Mastering