Developing Boston for 1920s Call of Cthulhu
While Chaosium has produced sourcebooks for many locations in Lovecraft Country (Kingsport, Arkham, and Innsmouth) as well as non-fictional cities of the 1920s (such as New York City and New Orleans), the nearby city of Boston, while being featured in many adventures, has never received its own full sourcebook.
The most common era for my Call of Cthulhu games has been in the 1920s - some campaigns have been in New York City while others have been in Lovecraft country. For those in Lovecraft Country, many adventures have been in nearby Boston. Over the years I've slowly been building up some expertise in the area and I thought it might be of interest to others gaming in the same setting - whether with Mythos horrors or a purely mundane game.
First, let us investigate those official sources. While Boston has found its way into many adventures, I can think of two sources where it has been given a fair amount of detail. First, The Unspeakable Oath double issue 16/17 has a reference on Boston. Second, "The Art of Madness" by Brian Courtemanche, included in Chaosium's The House of R’lyeh adventure anthology has a brief but very well-done overview of Boston and the adventure itself really dives into details of Boston - institutions, neighborhoods, ancient smuggling tunnels, etc., all of which are real - I highly recommend it.
Going into Lovecraft, I find "Pickman's Model", the inspiration for "The Art of Madness", similarly goes into nice details of Boston.
Contemporary tourist books are often available, sometimes digitally, other times from eBay or Amazon. I like the 1920 edition of the Rand McNally Boston Guide as well as Boston: A Guide Book by Edwin M. Bacon from 1922. Both books assume someone unfamiliar with Boston - though obviously both are giving a focus on tourist destinations and not on neighborhoods or the seedier aspects of Boston (no speakeasy guides here...)
Tufts University has Boston Streets: Mapping Directory Data. This project includes very detailed maps of Boston in 1928. Historic Map Works has similar data. I find their 1922 Maps of Boston handy as my games have tended towards the earlier part of the decade. They are a great source of historic map data for many cities - I've used them for New York City in the past.
The non-fiction work Dark Tide: The Great Boston Molasses Flood of 1919 by Stephen Puleo is a great read, really capturing the feeling of post-World War 1 Boston. I've reviewed that book previously and have used the event in my game to great effect. Puleo has also written books on the Italian immigrants of Boston (The Boston Italians) and Boston from 1850 to 1900 (A City So Grand).
I'm sure there's more sources - and I'll gladly add to this list if people have suggestions - but hopefully this will be of some value.