It's possible, but it was a book in a relatively unusual niche that I have a hobbyist interest in. It was a book about the archaeological and linguistic study of proto-Indo-European people (PIE) and their origins and spread by means of horses and the, at the time novel, wheels. I had previously purchased a book or two on linguistics from Amazon so perhaps there is only a small set of popular books in that category for their recommendation algorithm to pick from.
Ah, yeah, that would explain it. Thanks!
Well there was this sometime ago: https://news.ycombinator.com/item?id=14406583
Though that was Google.
That is creepy. Any way it might've been more than just a coincidence?