I came up with “Moore’s Law of Literature” about a year ago.
It is quite simple, and so far, always accurate.
Here’s how it works. I take the total number of pages a book has, so in the case of The Last Days of Socrates by Plato, we have 183. I divide this by 2 so 91.5. If the total of my marginal notes exceeds 91.5 then it is a formative book. In the case of The Last Days of Socrates I made 102 marginal notes.
Fortunately, even the modern books I’ve read this past year have all passed the test. If I went back over a lifetime of reading there would be many books that would not.