Although it's still listed as a beta book on the Pragmatic Bookshelf web site, Mike Clark's Advanced Rails Recipes is finished and getting ready to ship. Thanks to Pragmatic's excellent beta program for books (you get to read in-progress PDFs while the author is finishing the book), I've had plenty of time to work with this one already.
My verdict: the book is a winner, and its very existence says good things about the growing Rails community.