FWIW, I actually live in the Boston area, and I think a "Secrets of Boston" sourcebook would be a really compelling read!
For contents, most of it has been mentioned above, but an overview of the public transit system, including period pictures, would be great. (We know Pickman painted pictures of a "Ghoul Invasion" from under the subway; what would the station have looked like at that time? And there was a GREAT adventure in an old White Wolf Magazine focused on one of the "Abandoned" subway stations.)
And, of course, we'd need some cool scenarios.