This sounds like a fantastic idea! There are so many great locations in Boston that you can write and get inspired from.
You might take a trip up to North Conway and explorer some of the Mountain Passes and also travel some of the ferries to the Boston Harbor Islands.
You must stop at Georgia's Island where Fort Warren is and spending atleast a half day walking around, go into the middle of the fort - nice grassy green. This supposedly is a very haunted place at night and on the list of most haunted places. Confederate soliders where held here during the Civil War.
The Fort itself was designed by Sylvannus Thayer. Do a little google look up on him, quite an interesting fellow himself.
Another less known legend is that of the Westford Knight. Legend has it that the templar knights landed on the shores of Massachusetts and traveled and explorered inland. Who knows, perhaps they buried their huge treasure of gold there!
There is an a rock known as Sinclair Rock that supposedly contains an inscription by the templar knights. It is not acknowledged by historians because it can not be verified.
Here is a wikipedia article:
Good luck and be sure to let us know when that book is done. Would love to have a read!