The Pragmatic Programmers, an excellent publishing house for technical books and resources, has started shipping TextMate: Power Editing for the Mac. TextMate is, hands-down, the best editor for the Mac - if not for any OS. Just ordered the book and PDF combo myself.
I'm not exactly sure what the 201 page book actually covers but if the table of contents is any indication, it's going to be a worthy addition to my technical books collection. I've been told the book covers some of TextMate's hidden functionality.