Maybe I was wrong to call Nunn's Chess Endings comprehensive. It isn't really a systematic treatment of theory at all, but rather a collection of often quite tactical situations taken from practical play, which require accurate calculation and still illustrate plenty of common themes. So it should be a good add-on to a study of, for example, the Dvoretsky, Emms or Averbakh/Kopaev material, since it duplicates very little of it. Of course there are other sources for tactical rook endings, like Minev (much more basic, but good) and Van Perlo (which I haven't seen, but everybody seems to love it).

I notice most of the Rook endings section of NCE 2 has quite reduced material, up to just 2 pawns vs 1. But there are some 70 pages with more complicated examples at the end.

If you're looking for "untheoretical" material with up to 6-7 pawns each, maybe you'll need to look for examples in more strategic endgame books, like the ones by Müller/Pajeken, Lars Bo Hansen, Hellsten and Shereshesky (including his two with Slutsky). Most theoretical books, like Emms and Averbakh/Kopaev, go up to 4 pawns vs 3, with just a few examples of endings with more pawns.

I haven't studied any book on rook endings in depth, but of the ones I own or have looked at, the most complete/comprehensive are Averbakh and Kopaev's volume 5 from Comprehensive Chess Endings and Nunn's Chess Endings, volume 2.

Nunn's other book Secrets of Rook Endings isn't really comprehensive, since it's exclusively on Rook + pawn vs Rook, though Nunn helpfully points out the most important positions for practical play in every section, so it's possible to study only those. The Averbakh/Kopaev volume now comes with lots of "potentially out-of-date" warnings. Convekta released Comprehensive Chess Endings with updated analysis on CD some time in the 2000s, but it seems to be impossible to find now.

Honestly I suspect you'd be on a strong GM level in rook endings with the Dvoretsky's Endgame Manual chapter plus the Emms and Korchnoi books (though these two should probably also be checked against the most recent tablebases), if you knew all of that by heart.

Seeing these references to "(really) mastering" Rook endings, one thing I think of is the fact that John Nunn wrote a 350-page book on R+P vs. R.

DenVerdsligeRejsende wrote on 07/17/15 at 02:50:08:

You can probably look at your last hundred games or so, estimate how many half-points you would have gained from knowing rook endgames a bit better (and applying that knowledge successfully!), and figure out what the effect would have been on your rating.

I note that all the rook endgame books are filled with terrible blunders by 2600+ rated players, who somehow got to that level while still making big mistakes in relatively elementary positions.

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That seems like a good enough reason on its own to study them further!

It is by Olms and says 2nd edition 2002. I guess the first was the german edition. I think I got it at Schackhandeln in Stockholm quite a long time ago.

Is that Korthscnoj book a different book, or same as the German book Praxis des Turmendspiels? I saw a German book by Olms long time ago as a junior but never bought it. Any endgame book by Kortschnoj is bound to be great. I wonder where they sell that one.

What is a good book that is focussed exclusively on rook endgames, that can be useful for 2300+, going from the basic known stuff like Philidor and Lucena but also having complicated examples with rook + 5 pawns vs. rook + 5 pawns? Basically rook endgames that just came from a middlegame. 

As a Caro-Kann player, stuff like double rook endgames like 2R6P vs. 2R6P is not uncommon. Which kind of book discusses this stuff in addition to the simpler theoretical positions?