Lovecraftian purists might only look at the works of the author as their sole point of canon for the mythos, and that is fine, but it should be noted that throughout his prose writing days, he shared many terms and ideas with other authors of the time. This group, who became known as the "Lovecraft circle" included August Derleth, as well as a few others, and Lovecraft did indeed encourage Derleth's writing.
While many scoff at how Derleth changed the mythology to suit a "good vs evil" take, as opposed to the mysterious and ambiguous motivations behind the creatures in Lovecraft's own writing, much of Derleth's ideas have become just as deeply incarnated in the mythology as Lovecraft's original works. It wasn't just Derleth who dabbled in the mythos, however. Literally hundreds of writers have had a turn in Lovecraft's world, and the results have been at times convoluted and contradictory.
I want to mention also that I don't find myself a fan overall of Derleth's works, or the works of other writers who have dabbled in Lovecraftian lore. When I read Lovecraft's works, I get a distinct vision in mind of how he wants the story to come across, and personally when I read anything from the mythology, I prefer to stick to pure Lovecraft.
The Cthulhu Mythos Encyclopedia by Daniel Harms. I was skeptical of the volume at first, although very appreciative of the thought to pick it up by my wife. She likes to indulge my eccentricities if, at times, she never fully understands them. Despite my initial skepticism, I must say, I've been rather impressed with it thus far. My hat goes off to Mr. Harms for taking the time to pour over thousands of pieces of fiction, essays, and correspondence between Lovecraft and his peers.
It would be difficult, and probably nigh on impossible, to compile a truly consistent "encyclopedia" of the mythos, and Harms essentially acknowledges as much in his foreward saying:
...it represents one person's perspectives on the present state of the phenomenon known as the "Cthulhu Mythos" - a collection of fictional monsters, books, places, people, and other elements which weave together the works of Howard Philips Lovecraft and other authors through a stream of common reference- in all of its glorious confusion.The encyclopedia takes works of the mythos from a variety of sources, including the Call of Cthulhu RPG, as a frame of reference. It has various entries on the people and places which have shown up in Lovecraft's own stories, as well as those of his peers. Detailed descriptions of various monsters and entities throughout the mythology are present, as well as a list of all the people known to possess a copy of the Necronomicon in the fiction. I will say, that in no shape or form should this volume replace the original pieces of fiction which inspired it. Rather it serves as a good reference point for those interested in the mythos.
From a gaming perspective, this book is a gem. It is literally a treasure trove of ideas, plots, places, items, monsters, and people which could easily serve as inspiration for a great session of role playing, and not just in a Call of Cthulhu game either. I would say anyone interested in incorporating a little bit of Lovecraftian weird into their role playing, go out and pick up this book. It can be had on Amazon for around $15 for the softcover. And for those of you running actual Call of Cthulhu games, or games inspired directly from such (i.e. LotFP), the Cthulhu Mythos Encyclopedia is a must have.