I took a run down to the bookstore tonight to look at "Debian GNU/Linux Bible", on the strength of the recommendation on this thread. I found it rather dated, and I didn't think it went into as much useful detail as "Debian GNU/Linux 2.1 Unleashed". Admittedly, that's dated too, but I didn't find anything on Debian that's newer. The "Linux Administration Handbook" has quite a bit on Debian, though, and it's a lot more recent.