story identification - Identify this book in which time/magic vary locally from east to west

I saw a review of a book two or three years ago in which time and/or magic vary from east to west. It sounded fascinating, but I sadly forgot to write down the key information. I'm pretty sure that the review was on BoingBoing, but their archive search is not particularly good, and I'm turning up nothing.

This one should be a lot easier than my other stumper!