“The Moon Pool” (1918) by A. Merritt, in its original 17,000 word novelette version. The H.P. Lovecraft Encyclopedia states that Lovecraft…
considered the novelette “The Moon Pool” (Argosy [Argosy All-Story], 22nd June 1918) one of the ten best weird tales in literature; he disliked the later novel version
Basic .mobi (Kindle) and .epub conversions are here.
Sadly there appears to be no free audio-book reading, although Librivox has one for the later novel. The novel is apparently a rather poorly-structured combination of the original story with a six-part sequel, all of which was then abridged for book form. Merritt seems to have had the Elizabethan / folk tale approach to the ‘sanctity’ of his texts, freely hacking them about and adding to them in order to fit each subsequent appearance. Science-fiction: The Gernsback Years points out that few early SF fans ever got to read the original 1918 Argosy All-Story version, reading either the novel (1919) in book form or the Amazing Stories magazine reprint of the novel in May-July 1927. Science-fiction: The Gernsback Years claims the original story was not reprinted from 1918 through 1970, but I have found a record of what appears to be a reprint of it in Famous Fantastic Mysteries, Sept-Oct 1939.