Not quite sure, but if you look for endgame books, Silman's complete endgame course might be better than Nunn's understanding chess middlegames.
For chess theory, I like Dvoretskys Endgame Manual. It's very difficult, but I just did the theory, and played it out with my friends, not wasting time on the very difficult exercises.
Endgame Strategy by Shreshevsky or something like that is the best for technical endgames
I haven't read Nunn's, so I can't give a comparison. I can say that Silman's is good. Many say it's his best work. It is presented in endgames you should know for different rating levels. Unrated to 999, 1000-1199, 1200-1399, etc. all the way up to 2200-2399. It is a book you can pull off the shelf and work through the section you're in, and then put it back on the shelf again. Rinse and repeat when you hit a new rating range.
I've only actually seen the one I'm working in now so my opinion is limited somewhat because I don't really have something to relate it against.
I'm currently in Muller's "Fundamental Chess Endings". So far I have found it helpful, not just for the endgame, but it seems to reenforce how pieces work with and against each other in other parts in the game. My "intuition" in the middlegame seems to have also improved by working in it.
If your a beginner or want the basics Karsten Muellers Endgame DVD 1 otherwise his DVD's 2-7 at club or 8-14. All are excellent.
Which is the best endgame book? Silman's complete endgame course or Nunn's understanding chess middlegames?