Ballantine published a pbk series back in the early-late 60's called "Ballantine Adult Fantasy" which resurrected many rare titles and brought them back into print.
Lilith, by George MacDonald, was one of them. It became very popular for a while but seems to have slipped back into obscurity. It's a remarkable book.
A Voyage to Arcturus, by David Lindsay is another.
Although Tolkien's stuff was part of the series, the rest of the genre wasn't as choked with Medieval European mythology the way it is these days. Wizards, demons, trolls, fairies, and especially elves with pointed ears were pretty rare, thank goodness.
Some other titles that made their first appearance in paperback as a part of this series are not so rare now:
The Gormenghast Trilogy, by Mervyn Peake
The Worm Ouroboros, by E.R. Eddison
The Dream-quest of Unknown Kadath, by HP Lovecraft
I spotted the following when I went to the bookshelf to make sure I had all my spellings more or less accurate. They weren't part of the Ballantine Adult Fantasy series.
Back when I first read them they were pretty rare titles. I don't know what their status is now.
Hidden World, by Stanton A. Coblentz. The background plot is about two large underground civilizations, beneath the midwest (? I think) that go to war against each other. It's actually on a pretty big scale considering the location. Both civilizations share a vast cavern at least hundreds of miles across.
In the foreground it's a social commentary, which was Coblentz' forte. The cultures are complete lampoons of Western Civilization. It's funny and fascinating.
Planet of Peril, by Otis Adelbert Kline
Prince of Peril, ibid
Port of Peril, ibid
The Swordsman of Mars, ibid
The Outlaws of Mars, ibid
Kline wrote these "Planetary Romances" at the same time Burroughs was doing the John Carter of Mars series. A legendary feud between the two authors has been discounted. They were probably both just capitalizing on the popularity of the pulp genre in those days.
The timing suggests that Burroughs started writing the John Carter series first. Kline then wrote his Venus series, the three ...of Peril novels. Burroughs followed with a Venus series of his own, which Kline in turn followed with his two Martian novels!
Kline wrote some "Jungle" novels as well. The one I read is Jan of the Jungle.
Kline was one of the first editors of Weird Tales magazine, which eventually positioned him to become a literary agent. One of his early clients was Robert E. Howard.
I really took a tour of the bookcase on this one. Thanks for the motive!
Have a good one,