Lots of book sites have recommendations. E.g. "Readers also enjoyed" on Goodreads and "People who bought this also bought" on Amazon.
They're anonymous-ish (in that you don't have to be logged on, though I'm sure tracking cookies gunna track), and they're direct, but they're only for one book at a time.
And in which other sites you input a list of books and get a list of recommendations?
Like this one: anonymously, directly?