M.J. Medwick asked:
I’m interested in your research process for emulating period-specific media – are there particular online resources the authors tend to favor, or is much of that work done “boots on the ground” in libraries and archives?
Speaking personally, I used a mixture of my personal library, internet research, and a few jaunts to the libary.
My personal book collection includes:
- My mostly complete collection of Call of Cthulhu titles. Great for finding scenario connections and to see what has come before. In this case I made use of Secrets of New York, The London Guidebook, Green and Pleasant Land, Kingdom of the Blind, The Cairo Guidebook, Secrets of Kenya, Terror Australis, Delta Green: Countdown, and (of course) Masks of Nyarlathotep. There were probably more…
- The Cthulhu Mythos Encyclopedia by Dan Harms. This is the one book to get. It summarizes all the major
(and minor) elements of the Mythos plus a helpful guide to the relevant fiction and it is where I turn first. - Ex Libris Miskatonici by Joan Stanley. A fictive catalog of the Miskatonic University library special collection, it was one of the primary inspirations to my approach to tomes in the Companion, treating them as real books, not just generic bits of treasure that confer a few spells and points of Cthulhu Mythos skill.
- Period guidebooks (Baedeker, Rand McNally, Muirhead) to London and New York. Those before 1923 can sometimes be found in Google Books, which is also a great place to check. Also a 1927 World Gazetteer, basically a very small atlas with some national stats in the rear; I used this to double-check placenames… which are often different than modern ones, especially outside of Europe and the Americas. Ditto for archaic transliterations in Asia (Wide-Giles in China, for example).
Beyond this on specific points it was a lot of internet searches reenforced by trips to the library as needed… the internet, despite certain advantages, has its limits. Google books is a helpful resource, especially since books up into the 20s are now in the public domain and therefore are available in full text, searchable form.
Finally, the Library of Congress photo archive is fantastic, with lots of high resolution TIFF images available to you the tax payer. Similar archives exist for the UK and Australia. I’ve used it as the source for the image on this page, for example.
Any more questions?
Tomes in Progress 
Turns out there’s a novel that includes a number of the characters (Cthulhu’s Dark Cults) though many of them are set after Masks. Haven’t read it yet but it could make for a good source of inspiration.