I've started reading through the Hippocampus Press edition of the letters between Clark Ashton Smith (CAS) and August Derleth. While much of the correspondence concerns mundane details of story submission and editorial whims, there are some interesting observations from CAS in these pages on the art and practice of writing.
CAS began an English translation of Charles Baudelaire's Les Fleur du Mal in 1929. He never finished that work, but in a letter* to Derleth from 1931, he expressed a concise summary of Baudelaire's particular appeal:
Funny - I seem to have lost interest to a large extent, in French writings and translations. My Gothic side has been cropping out more and more, so that the French genius seems rather too earth-bound and concrete and realistic for my present taste. Baudelaire, it is true, has a sense of the gulf; but it seems to be an internal gulf, rather than the true cosmic vastness which I find in Poe and Lovecraft.
That notion of "an internal gulf" is spot-on, and captures in a very few words the essence that drives Baudelaire's poetry, and makes him unique from his great muse Edgar Allan Poe. And of course, CAS' preference for the "cosmic vastness" of Poe and Lovecraft makes perfect sense in light of his own creative output.
*See letter #46 in Eccentric, Impractical Devils: The Letters of August Derleth and Clark Ashton Smith published by Hippocampus Press.