I can't say much about the 1999 version of John Nunn's Secrets of Rook Endings, because it is so specialized: there are 342 pages of rook-and-pawn versus rook. Nunn generated every possible ending of this type using a computer program, thus ensuring an error-free book(!). His task was to find themes of interest in this mass of material, and an incredible finding he cites is that there are 209 examples of mutual zugzwang in these endings, all of which he lists. This is a massive, futuristic project, but I'm not sure how many people will find it compelling, the exception being endgame fanatics who want to achieve certainty about these deceptively complex endings.

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