It's a bit like the problem of comparing great composers and great music -- for example, it doesn't really make sense to try to decide whether Bach was a greater composer than Beethoven or any other great composer. Thresholds are a little better: "above the line is 'great' ", etc. But then drawing the line is tough.

In this list I didn't include quite a number of books I think of as "great" -- instead I put in readable books that I thought would help the target readers.

Just to pick one as an example: I think Newton's "Principia" is a great book. It's not easy to read, and it would not have been a good choice for this group, but it is above any threshold I would draw.