Since I recently gave a high rating to (the early) Mad Scientist Club books, Amazon has started recommending to me all sorts of other books that I loved when I was roughly the age I discovered those at. The Alvin Fernald series, the Great Brain series, Homer Price, a few others. Not sure how many of them I'll buy, but I was amused by the clarity of the correlation :-)
It might be just my imagination, but I think Amazon is being insidiously clever with their discount pricing. They offer free shipping for orders over $25, and if your current order is less than that, they tell you how much you need to add to make up the difference. I get the impression that their discount structure is designed to create lots of books in the $11-12 price range, so that 2 isn't *quite* enough, and also a lot in the $7-8 band, so that if you're in that price range, 3 won't do it. Entirely anecdotal, and maybe imaginary, but there it is.