There are many, many fine products that come out of the HPL Historical Society, from their Dark Adventure Radio Theater audio plays to their physical props (I rather badly want their Innsmouth scrimshaw shark-tooth). But what I really like are their scholarly monographs. They’re written in an academic but lively style, and as if they were in the actual world of the Mythos.
The latest one — The Discovery of Fragments of Kitab Al-Azif at Harran — is I think of particular interest, as it would take a gamemaster roughly half an hour to construct a full-fledged adventure, based on nothing but what was in the pamphlet. All you’d really need is your favorite monster from the game, and you’re good to go. There’s even an easily-detachable game prop, with a helpful colored map on the other side!