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The page might become useful although everybody might have somewhat different opinions about rank ranges and KGS ranks are not particularly suitable.
The page should get more links to it.
As useful as the page is, it is sort of discriminating by hiding information about other books. What about books containing 1/3, 1/2 or 2/3 theory and the rest is filled with problems - or books containing more theory than problems but more problems than some of the problems-only books? Do we really need an extra page about Estimated Mixed Book Difficulty about books with a good balance of problems and theory, or can't we just alter the declared intention of the page to include mixed books? Would we want a third page for theory-heavy books, to provide on average more up-to-date information than my ISBN rating list?
I think it is fine to have mixed context books here, as long as they have a structure making them suitable for using the problems for bulk repetition training. If the book has single problems interspersed here and there with the theory to highlight the theory, it is not suitable IMO, but if e.g. every chapter has a problems section of 20 problems at the end, then I see no reason not to list it here.
So IMO the reader should able to quickly find and do the problems in the book in bulk while ignoring the theory, because I think readers of this page are looking for a "what can I buy at my level to do bulk repetition training with" reference list.