Robert LoGrippo is responsible for the garish, blood-drippy covers from 1976. The fifth Pinnacle paperback is from 1983 and contains all the stories from the two-volume set. Can't even recall which one(s) I owned, and I haven't reread even "The Great God Pan" in over 15 years. Surely it is one of the seminal tales of classic horror.
In 1972 the highly-regarded Ballantine Adult Fantasy line published The Three Imposters (1895). This novel - with Boschian art again by LoGrippo - in episode form includes two of his most influential stories, "The Novel of the Black Seal" and "The Novel of the White Powder." Below you can see the British paperbacks with those titles, from Corgi in the mid-'60s; love the covers by Josh Kirby.