First I suppose I should say what this book is not. It is not a bibliography of all the books, chapbooks, stories or web fiction that use or are about the Cthulhu mythos. With the explosion of mythos sites on the internet and the rapid pace of publication by small presses, such an endeavor would be out of date before it ever hit press. Mr. Harms cites Chris Jarocha-Ernst's A Cthulhu Mythos Bibliography and Concordance from 1999 as useful in this regard. I have used Glynn Barrass' similar chapbook from Rainfall Books as well. A continuously updated online reference would be invaluable for collectors and fans but alas there is nothing definitive. I really enjoy EP Berglund's site, The Reader's Guide to the Cthulhu Mythos ([...] but it has been under construction for some time now. As far as I can tell, The Ultimate Mythos Book List ([...]) has not been updated in 2 or 3 years.
What this book does provide is a detailed description of those entities, characters, tomes, places and critters that populate the worlds of Cthulhu Mythos fiction. Mr. Harms makes no pretense about being comprehensive; monsters that may appear in only a single story will not show up here. Fortunately, Mr. Harms lists his own criteria for inclusion so there is no ambiguity: all entries from the second edition, entries from the first edition where there was a groundswell of popular demand, and things found in two different works by two different authors, or appearing in major Cthulhu Mythos novels. Thus you certainly find the Hounds of Tindalos but you won't find T'loal (not that you would want to; the novella was abysmal). There is likely quite a bit of RPG material that has been excluded as well, although I found a citation on Delta Green.
I have a copy of the limited hardcover edition, a lovely book signed by the author. Art on the slip cover was provided by Malcom McClinton, an was quite nice, with some cephalopoidal thing probing about a library. I don't think Mr. McClinton has been active on the mythos art scene very much, but I hope to see more of his paintings in the future. Page count was a generous 382; materials used in the book were of highest quality and production values are flawless. The paperback is a bargain, with the discount and free shipping offered by Amazon; I have been pleased with all of my paperbacks from Elder Signs Press.
Several features stand out. I really enjoyed Mr. Harms' detailed introduction, with his description about the creation and growth of the phenomenon that is the Cthulhu mythos. There was, I think, a very even handed description of the role and contributions of August Derleth, always a contentious subject for mythos fans. I would have liked to see more about Lin Carter, but that's just a personal bias. Perhaps the least useful (or perhaps least likely to be used) part of the introduction was the guidance offered to authors about how to employ the trappings of the Cthulhu mythos in their stories and books. Like anyone is going to allow themselves to be limited! The appendix about the Necronomicon was quite good, as was expected as this is a special area of interest to the author. I skipped around reading citations about some of my favorite beasties and people. One of my bench marks is how does an author come to grips with the Outer Gods, the Elder Gods, the Old Ones, the Great Old Ones and the Great Race, etc. No complaints here. I enjoyed the treatment of all of these topics. I also don't think Mr. Harms intends these entries to be definitive depictions, of Cthulhu for example (In the short story by Neil Gaiman, I, Cthulhu, there was a pithy description of Cthulhu's 'birth' and its reproduction, which I prefer to what is listed in The Cthulhu Mythos Encyclopedia). Instead he is offering in one place, a description of how these entities have been described, depicted or used by their creators and a large comunity of writers.
So who should be most interested in this book, other than fanatics like me? I think authors who are trying to keep all these names and places straight would find it useful. New fans to the mythos who are daunted by all the cross referencing that happens between mythos authors (and that is part of what makes mythos fiction so cool for readers) now have a scorecard to identify all the players. Role players now have a handy compendium (alas without pictures) to add depth to their campaigns. And of course collectors must have it all.
Bravo and thank you, Mr. Harms!