As a librarian, some of these suggestions look very useful and I’m going to share them with the rest of the Reference Desk staff.
But if your library has access to EBSCO’s NoveList database, it is the greatest book recommendation engine hands down. Due to budget cuts, my library system recently had to get rid of it, but ThisOneNext and Literature Map at least give me a starting point for something similar.
I just typed Stephen King into the Literature Map; they placed J.R.R. Tolkein right next to him, and out on the side Clive Barker is right next to Jane Austin! 
Wow - and Ayn Rand and John Steinbeck? I’m struggling a little with some of these.
I think this is a case of “a rising tide lifts all ships”… Yeah, Amazon would prefer that you buy books from them, but anything that gets people reading more is good for Amazon in the long run. Maybe you won’t be able to find a copy elsewhere, so you’ll buy it anyway. Or maybe you’ll read the book from the library, and recommend it to a friend, who then buys it. Or maybe you’ll just come to like Amazon in general for helping you out with book recommendations, like Macy’s in Miracle on 34th Street. And after all, Amazon has listings for books they don’t actually have in stock: Surely, they don’t expect to sell you copies of those books.
Well, this thread has helped me figure out what to do on a rainy Saturday – rate 1128 Amazon transactions! We’ll see if that helps.
More likely it’ll result in more buying. I need to see if some of these authors have written anything new. And I need to find a copy of Slaves of Solitude – I ordered it months ago from Amazon but they were unable to deliver. Maybe I can find it used.
I rely on the reading threads here for recommendations. After awhile, I know that if X likes a book, I will too.
I like the website movielens - it’s hosted by a research group the University of Minnesota. It doesn’t just give you recommendations, it guesses how you would rate the movie. You enter ratings for movies you have seen (from 0 to 5 stars in 0.5 star increments), and it suggests how you might rate other movies, based on ratings from other people in their database who have similar tastes in movies. I find that the ratings they guess for me are usually very close to the rating I give a movie after I have seen it. From their FAQ
Try out bookwonk.org. It uses optical character recognition to actually read books and analyze the language. Then it recommends books that are linguistically similar to your favorites.
-FG
Please check dozen.softover.com - our attempt to make something that works 
I’ve told Amazon countless times that I already own Mansfield Park. It doesn’t bother me overmuch that they keep wanting me to buy Emma, which I also own but that they keep recommending me other editions of Mansfield Park. I enjoyed it once and read it a second time. I doubt I’ll be reading it again because I have way too many books on my To Read list but if I do decide to read it again, I’ll re-read the copy I already own, not buy another one with all the same words but possibly a different cover.