I plan to visit Boston this year; I'm looking to hit up the historical sites, and trying to find a good guide of all of the sites. Is there one out there online, or a good one I can find or buy when I'm in the city, not including all of the tours they offer?
A good starting point is the Freedom Trail -- it's a nice primer on Boston history... They also offer walking tours, I believe. The trail includes:
Beyond that, depending on your interests there are numerous other options. You could visit the Cambridge University campus (easy ride on the metro), Fenway Park, Newbury Street (some typical Boston architecture, plus a decent choice of shopping & restaurants), etc.
The Celebrate Boston site is a good resource for what's happening in Boston and what's worth seeing. I know that you mentioned historic, but worth mentioning Arts Boston for additional arts-related listings.
Also, have a look at MassHome's Boston Page. Not very pretty, but has lots of links to different tourist sites in Boston.