Fritz Lieber, Moorcock, Burroughs, and the entire Arthurian cycle, Greek Mythology, Norse Mythology, and even Chinese legends (where do you think Gold dragons came from? Original D&D) and I think McCaffrey's Pern ... speculative fiction of all sorts fed it. (Lovecraft => mind flayer was apparently confirmed by E.G.G.).
That squares with what I recall
Weirdly, I did this for a few years when I introduced my kids to D&D. But as we oldsters usually do, what I actually ran was a mish mash of multiple editions. Mostly B/X with some of my AD&D 1e world stuff in it also.with people who transitioned from AD&D, 3.x. or 4th edition to BECMI being a drop in the ocean.
I got B, E, C and M off the shelf, but I also got the whole suite and some neat maps from DTRPG .pdf recently.Since then, WotC has made D&D's back catalogue more accessible in PDF format. So if BECMI has grown a fat tail due to digital distribution, I wouldn't know.