When is a book finished? When the authors cross off every item on the to-do list? Uh uh---there's always errata, especially in this age of beta releases and preprint e-copies. What about when the first printed and bound copies arrive in a box smelling faintly of ink and travel? Same answer---in an era of print-on-demand, publishers can get a revision out almost immediately (I'm actually surprised the revised 13-digit standard for ISBNs doesn't include version numbers.
All that aside, there is something special about crossing off the last to-do. We're inches away from doing that for our intro to computer science using Python; Daniel, Steve, and the rest of the PragProg crew still have to create an index, tidy up the typesetting, and all that, but after all this work, it's nice to have some kind of end in sight.
Update: Paul Gries just closed the last to-do. Yay!