There are three main sources. For books, authors, stories, and games, you'll want "Cthulhu Mythos Bibliography and Concordance". "A Guide To The Cthulhu Cult", from Armitage House, is a fictional account of the origins of the Mythos from ancient times to the present day, and is very interesting. Last, there is "The Encyclopedia Cthulhiana", a thorough, alphabetical list of various creatures, gods, tomes, personalities, etc., from the entire pantheon of Mythos material, along with a brief cross- referenced definition of each. This was published by Chaosium, and is available in an original edition and a revised/expanded second edition.

Ai! Ai! Ai!

You know, somehow I don't think I'd like to read a Cthulhu Concordance. With that mythos, NOT knowing is half the fun.

And a lot safer.

Yeah, but then you can't cast a spell to turn yourself invisible, that accomplishes it by turning you into an invisible monster. When you do it right, that game's SO creepy.

Not to mention it's a real pain when MJ-12 wetwork squads or DG cells kick down your door.

I actually mentioned this in another thread, but thought it would fit better here. Richard L. Tierney's long delayed mythos novel, The Drums of Chaos, is now finally in print! I've only had the oppurtunity to glance at it, but here is a (very) brief synopsis:

WARNING: PEOPLE WHO GET OFFENDED EASILY, DO NOT READ!

The being variously referred to as God, Yahweh, Allah, etc., is in fact the Outer God Yog-Sothoth. Jesus of Nazereth is the progeny of Yog-Sothoth and a human woman, a la Wilbur Whateley from The Dunwich Horror. Simon Magus and friends, including a time traveler named Taggart, must stop his nefarious plans. Just in time for Christmas, to. Coincidence? Salman Rushdie, eat yout heart out!

Also in print is a third revised edition of The Encyclopedia Cthulhiana, now retitled The Cthulhu Mythos Encyclopedia. It is available as a trade paper back, but I splurged and bought the limited edition hard cover, signed by Daniel Harms. Both of these may be ordered through www.mythosbooks.com

EDIT: btw, Drums has really wicked cover art.