Sam Moskowitz assembles a dandy 1971 anthology of stories by major horror and fantasy authors that had not been previously anthologized and puts it together with copious and useful biographical and bibliographical notes.
The genre gem here (at least for me) is "The Challenge from Beyond," a multiple-author story from the 1930's in which each writer wrote a couple of thousand words and then passed it on to the next writer. And what a group of writers -- H. P. Lovecraft, C. L. Moore, A. Merritt, Robert E. Howard, and Frank Belknap Long drive this car all over the road, off the road, and upside-down, in their own distinctive stylistic and thematic voices. It's certainly not a great story, but it is a hoot -- especially when Robert E. Howard (Conan) goes off on an almost stereotypical Robert E. Howard tangent in his section, leaving Long to figure out how to put everything back together again to end the story.
The rest of the collection has its joys too, especially to someone steeped in the genres and writers of fantasy and horror. Ray Bradbury's first published story and the revision of that same story he did with Henry Hasse isn't particularly good, but it's a great window into the author's brain in its earliest stages. The entries from Marshall, Merrit, Norris, and Sabin are all fascinating rarities.
C.L. Moore's solo entry, "Werewoman," is a dream-like, uncharacteristic entry in her pulp-space-hero Northwest Smith's adventures. The weird night-journey of "Unseen - Unfeared' by Francis Stevens has gone on to be anthologized numerous times since 1971 for its strange mix of science and the supernatural. And "Hand to Mouth" by the major and short-lived mid-19th-century fantasist Fitz-James O'Brien ("What Was It?", "The Diamond Lens") seems like a novella written 50 years too early. It, too, is a night-journey of dreams and nightmares, almost surreal or even dadaist in its sensibilities. In all, highly recommended.