My guess is a typo that didn't get caught in proofing.
Having been in the position of reading blue-line copies for a publisher, there are always errors of this sort. Always. Drop an e-mail to the publisher. If they've not caught it already, they'll stick it in their file of things to fix before the next print run.
(Blue lines are the proof copies that are looked at prior to the print run. They really aren't meant for catching mistakes that the proofreader misses, but it does happen--bigger errors do get fixed. It's real purpose is to make sure that pages are in the right order and that digital files are working properly at the printer.)