Which Ancient One did Tulzscha serve? Did Bast oppose Nyarlathotep? Which Ancient One is associated with Kingsport?
As a longtime player of the Arkham Horror boardgame, I try to thematically associate an Ancient, its Herald, any Guardians opposing it, and the city the Ancient One had activity is in. Unfortunately, the Arkham Horror game has no background on these entities, and it's up to the players to research "Who's Who" in the mythos.
But this is a large task. Not only does the Arkham Horror boardgame series utilize entities, locations, creatures, and items from the original Lovecraft stories, but it also draws from other authors who contributed to the mythos as well. And, not only would there be quite a bit to read to understand these elements of the mythos, many of these stories aren't really worth reading.
The Cthulhu Mythos Encyclopedia, then, is an invaluable resource for us theme junkies who really do care which Ancient One goes with which Herald in which expansion set location. And, of course, any Keeper who needs to look up a diety, person, creature, or item of the Mythos will find it useful as well. As someone who's tried to use the internet for this information, I've found internet sites and the Wiki woefully inadequate.
The information is organized by mythos entity or item and is in ePub format, under 1 MB. In addition to encyclopedia entries, the Encyclopedia has a short background of Lovecraft and the mythos itself. Very useful and very entertaining.
[5 of 5 Stars!]