Walter Hooper's "C S Lewis: Companion & Guide" (if you can find it in the library or order it) has several discussions of Lewis and mythology. And "The C S Lewis Readers' Encyclopedia" (which I wrote for) has a good article (by Wayne Martindale of
Lewis mentioned mythology several times in his writings, and some of these are:
1) In "The Weight of Glory" (chap 5) he mentioned mythology and paganism...
2) In the same book (chap 1) he had a quote about mythology and poetry...
3) In "God in the Dock" (pp 57-58, 66-67, 83-84, 132) he mentions mythology as the precursor to Biblical religion...
4) In "Surprised By Joy" (chap 7) he refers to Christianity as the Christian mythology... In the same book (chap 5) he mentions that in mythology he was almost sent back to the false gods to acquire some capacity for worship...
5) In "Reflections on the Psalms" (chap 10) he mentions the death and rebirth pattern in mythology as not accidental and that it teaches the truth that humans must undergo some sort of death in order to truly live...
6) In his "Letters" (24 Oct 1931) he mentions that the desire for a "vague something" as seen in pagan mythology shows a first and rudimentary form of the "idea of God"
7) In "The Weight of Glory" (chap 5) he says that if Christianity is a mythology it is not the one he likes the best... (This was before he became a Christian; he said he liked Greek, Irish and Norse mythology better)