LibraryThing is already a wicked cool service but it's getting better all the time.
One of their latest features is LibraryThing Local which is “a new [LibraryThing] sub-site devoted to finding, mapping and
describing the world's bookstore, library, book fair and festival—as
well as all the readings, signings, lectures and other events they
host.”
Here is a fascinating post from one of the LibraryThing blogs looking at the distribution of bookstores v. libraries in cities across North America.