"Smoke and Spice" has already been mentioned. There is also a follow-up book by the same authors (Jamison), "Sublime Smoke"; also worthy of having, although not as good as the first. And of course, the usual caveat, "the times and temps are both too low".
In addition, Raichlen has a good book, "The Barbecue Bible". It's as much about grilling as about smoking, but many of the recipies can be adapted and in any event, it's a good book to have, if you're looking for barbeque books.
-- Ken