Nunn's Chess Endings volume 2 is actually the third and final book in Nunn's series. Understanding Chess Endings contains all the fundamentals that Nunn thought the student needed to master before diving into the study of complex endgames, and the diligent student would do well to read it first. Then Volume 1 focuses on all endings except those with a rook, and this book covers all endings with a rook.
Nunn's goal is twofold: 1) to convince the reader that tactics matter in endgames; 2) to equip the diligent student with most, if not all, of the ideas necessary to notice and apply those tactics in endgames. Notice that I have twice used the word "diligent" in this review. This book requires careful, slow study, not a quick read, and I don't think any other form of study will help much. All of the examples within have tricks and hidden points, so a careful read is bound to be more educational than a brief review.
Compared to the other two volumes, Understanding Chess Endings is a pretty good one-volume overview of all the different types of endings, and just about any player would benefit from reading it, all the way down to the level just above novice -- in tournament play, anyone with a four-number rating would benefit; in everyday parlance, an enthusiastic youth or local club player. Nunn's Chess Endings 1 covers a wide variety of endgames with the same intent -- teach players how to spot and use tactical ideas in the endgame -- but because of the breadth of material, the details are less overwhelming.
By contrast, Nunn spends the first 66 pages on several fundamental ideas in rook endings. The first, the "hesitation check," is a tactic I had observed before but which never had a name before. It refers to intermediate checks that cause the opponent to waste a tempo, and they are indeed crucial in rook endings. Then he covers the fifth-rank cutoff, switching the angle of attack of the rook, Lasker's method of driving back an opposing king, and several others. After that he dives into specific examples: 10 pages on all the nuances of play in rook and pawn versus lone rook endings, divided up by which file the pawn occupies; 40 pages on rook and two pawns versus rook, 30 pages on rook and pawn versus rook and pawn, 35 pages on rook and two versus rook and one, and so on. At each step he offers text ideas followed up by detailed variations illustrating the ideas, and sidelines (sometimes main lines) showing strong players mishandling what should be basic endings.
So diligent is the operative word here. Because of the need for thorough study, this appears to be the kind of book a club player (USCF 1500 to USCF 2000) might use to thoroughly train on this type of ending and try to move up into the master ranks. Other players, enthusiastic but less determined, may profit from The Survival Guide to Rook Endings. This book is also pretty thorough and detailed -- rook endings are complex, sorry to say -- but Nunn's book is definitely more detailed.