Thank you for the feedback.
If anyone is wondering, one reason that Robinson Crusoe and The Mysteries of Udolpho are not "apex books" is that they are both dominated by the Iliad.
* The Iliad by Homer (-700) — 432,847 ratings — 3.90 average
* Robinson Crusoe by Daniel Defoe (1719) — 297,130 ratings — 3.68 average
* The Mysteries of Udolpho by Ann Radcliffe (1794) — 15,120 ratings — 3.40 average
The Iliad is older, has more ratings, and has a higher average rating than either of them. It therefore "dominates" them in the sense defined on the page.
(I am not questioning the merits of these works, just explaining why they are not on the list.)