The others are completely unrelated cowboy books, by a different John Green.
Here's the page for judgments paradigm, in which judges rate how relevant a book is to the original on an absolute scale.
Nowhere does the related scroll panel make it clear that two of the books are part of a series, so the CTR on those two books would be just as high as if they were indeed the series introductions.
And if revenue is low, it's not clear whether it's because the suggestions are bad or because, separately, our pricing algorithms need improvement.
So to improve their recommendations, Amazon could try improving its topic models, add age-based features to its books, distinguish between textbooks and novels, and invest in series detectors.
(Of course, for all I know, they do all this already.) We now have a general grasp of Amazon's related book suggestions and how they could be improved, and just like we could quote a metric like a CTR of 6.2% or whatnot, we can also now quote a of 0.62 (or whatever).
Metrics like CTR, or even number of favorites and retweets, will probably optimize for showing quick one-liners and pictures of funny cats.