Currently printing and now on pre-order, Hideous Creatures: A Bestiary of the Cthulhu Mythos. Only 31 monsters, from Derleth and others as well as from Lovecraft, so it’s definitely not a cosmos-spanning encyclopaedia. However it’s apparently been in development for years and runs to 352 sumptuous hardback pages. Likely to be heavily illustrated and deeply informative about each monster, as apparently it dovetails with the Gumshoe-based Trail of Cthulhu tabletop RPG system. Since it’s for gamers there will also be a PDF download, albeit an expensive one.
Mentioned here because such in-depth books can be useful for writers, as well as for gamers.