Source material matters a LOT. Technical books that were originally created in an electronic format then printed or academic books that are "revised" every year get regularly proofed and reproofed. But these are the exception. For a large majority of books, I get the impression that the typical workflow is something along the lines of:
Author writes story up on Word, publisher extracts text, throws into template, prints books, dumps files onto archival CD, purges files from computer, goes on to next project, forgets where CD is stored. Author wants to reissue book, publisher grabs paperback, scans, OCRs, runs
spellchecker, throws back into template, etc.