If you want to pen your condolences, you should head over to Old North Church - yes, the place with the lanterns and all - starting Monday. The British consulate announced:
The British Consulate in Boston is opening a condolence book for Her Majesty Queen Elizabeth II at Old North Church in Boston (193 Salem Street). It will open on Monday 12 September and stay open through Wednesday 14 September from 10am to 4pm.