If you read a book, how many other related or similar books does it make you order? (Of wish to order, if you are budget constrained.) If the number is at least three or four, the book you read is almost certainly very interesting and worthwhile, if not always accurate.
Andrew Roberts’s biography of Napoleon made me want to read an additional biography of Napoleon, because it made his life to me more interesting. It made Napoleon’s period more interesting too. I might read a book on cavalry tactics as well, a topic I have never read on before.
Some books pretend to be the final word on a topic, but it is unlikely they succeed. If you don’t end your read with some additional book orders, maybe you need to ask yourself what exactly went wrong.
At times it is not a book order which is the appropriate follow-up. Say you read a book on Sri Lanka and you respond by going to Sri Lanka, well that counts too. Or a biography of Beethoven may lead you to more of his music, rather than to another book on his life.
If I apply the Amazon order test, the best book for me this last year was Michael Hoffman’s Where Have You Been?: Selected Essays.
Hofmann’s book wins additional points for chain effects, namely the books I ordered, as a result of reading Hofmann, in turn made me want to order further books. But chain effects are tricky. Following my read of Andrew Roberts, and then a follow-up Napoleon biography, will I read yet another life of Napoleon? That may depend on how good the follow-up is, and Roberts should not be held liable for that. Or should he? What should you think of a book which leads you to so-so follow-ups rather than to excellent follow-ups? A blog post which does the same?
What percentage of the value of a book is derived from the quality of the follow-ups it induces? Under plausible rates of discounting, for serial readers this could easily by eighty or ninety percent or more. (Could it be that actual book reviews are not consequentialist? Horrors.) How about a book review outlet which refuses to consider the books under consideration, but rather considers and evaluates what they will induce you to read next?
I would subscribe.