From Chris Sells' blog:

Then Mike read all 1300 pages, making sure that the copy editor didn't change the meaning of anything.

With the PDFs in hand, we both read the ~1000 pages again (the move to Quark puts in the final styles), looking for things that got messed up during the move between software packages or new things that we notice.

Egad!

I have a feeling that Charles Petzold is doing the same about now, since on 4/5 he said he had one more month before his 1000 page WPF book is due.