most comprehensive endgame reference?

Nothing is more comprehensive than the eight volumes edited by Averbakh.

Nice! Are they in English? Mine are in Russian, so I only looked at the diagrams.

Yes. All in English translation. Published in the 1970s by Batsford in England and Chess Digest in the US.

@23

They contain errors.

All books contain errors. The real question is:

Are the errors significant enough to discount the relevant value of the book? Probably not.

Back in the 1990s some people fed Rueben Fines Basic Chess Endings into a computer to determine the accuracy of the material. They found that it was 96% accurate. Pretty good for a lone man at a typewriter in 1943.

@25

"Are the errors significant enough to discount the relevant value of the book?"
++ Yes: Dvoretsky or Müller & Lamprecht are engine checked.
Checking Averbakh against the 7-men endgame table base reveals many errors: false evaluations, wrong lines. Why recommend something known with errors, when better is available?

But, these are far from comprehensive. I love Dvoretsky. No book has done more for my endgame, but Dvoretsky cannot pack into one volume what Averbakh and others (Chekover, Maizelis, Levinfish, …) presented in eight volumes.

For rook endings, a better choice might be the two volumes published last year. Theoretical Rook Endgames by Sam Shankland and Conceptual Rook Endgames by Jacob Aagaard. These are more thorough than Levinfish and Smyslov, Rook Endings in the Averbakh series, as well as more recent work by Dvoretsky, Müller and Lamprecht. They are also checked by computer.

Sam Shankland also has two books on pawn endings.
The OP asked for a comprehensive reference. The Averbakh series as a set remains the most comprehensive single source with instructive text, while Encyclopedia of Chess Endings remains the largest collection of positions.

Perhaps the OP is asking the wrong question.

Reading a section of Ilya Rabinovich, The Russian Endgame Handbook yesterday, I was astounded by the large number of obvious errors in notation. However, none of these interfered with my understanding of his analysis.

I don’t know. I thought it was a different packaging of the same series. Clearly, if Averbakh is the author of the rook volume, it is not the same.

At present, Internet Archive has Comprehensive Chess Endings. This a great service, as the books are impossible to find, but it is also a copyright violation. Internet Archive has been getting pressure to take some of these violations down, so don't count on finding it there.