As my current light-reading book,* I've been reading through one of Derleth's Solar Pons collections of Sherlock Holmes pastiches, THE MEMOIRS OF SOLAR PONS. While so doing I suppose I shd not have been surprised to find that at one point he inserts a Cthulhu Mythos reference, but the way he does so struck me as fairly unusual for Derleth.
The context comes in an occult library being offered up at private auction to six bibliophiles, collectors who specialize in occult tomes. The library, assembled by one Comte d'Erlette, is being sold as a whole, all or nothing. The Mythos comes in with the titles Derleth provides for the books making up the collection: twenty-seven items, those named being THE NECRONOMICON (Olaus Wormius's Latin edition), the Comte d'Erlette's CULTES DES GOULES, Ludwig Prinn's DE VERMIS MYSTERIIS, the LIBER IVONIS, and Von Junzt's UNAUSSPRECHLICHEN KULTEN. **
Except, as Derleth's detective points out, none of these books exist. The whole thing is a hoax, designed to get the collectors away from home at the private auction so their own collections can be burgled in their absence. The detective sums up:
All these books have a precarious existence
only in the writings of certain minor writers
of American origin, all apparently followers,
in a remote sense, of the work of Edgar Allan Poe.
(ibid.134; emphasis mine).
He then goes on to debunk the existence of any contemporary 'comte d'Erlette'. This is odd in a puckish sense because d'Erlette is a Frenchified version of DERLETH, which August D. used as a sort of pseudonym for his contributions to the Mythos
Given how Derleth devoted himself to promoting Lovecraft and his circle, primarily by publishing their works through Arkham House, it's a bit surprising to find him dismissing them as "minor writers". Certainly Lovecraft himself was given to self-depreciation, which often took humorous form when describing his works (cf. his letters to Clark Ashton Smith). Perhaps Derleth is simply adopting a Lovecraftian pose. Still, it's odd to find Derleth in his detective fiction creating a fictional character who asserts the spurious nature of tomes Derleth's characters frequently encounter in his horror fiction.
*contrasted with my current all-too-ponderous reading, GRAPES OF WRATH.
**Derleth, MEMOIRS OF SOLAR PONS (1951), "The Adventure of the Six Silver Spiders", p. 132