I bought this because I was looking to increase my chess playing tactical skills. That I could be assured of reasonable quality, given my general fondness and familiarity with Nunn as an author, settled it.
The book contains (surprise) 1001 puzzle positions, all of which are derived from actual games and most seem to be taken from the year 2000 A.D. and after. In each position a forced checkmate can be achieved for the side to play, black or white.
The book is divided up by chapter based on mating pattern: Deadly Doubled Rooks, Queen Sacrifices, Line Opening, Back Rank Mates, Death on the Rooks File, etc. After a brief explanation of the theme, Nunn gives a bunch of puzzles for the reader to solve, all of which are assigned a point value (i.e. difficulty); 1 point (~3 ply solution) up to 5 (~11 ply solution).
The overwhelming majority of puzzles are good (or at least adequate), but I found two principal objections. Firstly the stated point values for a few puzzles are considerably off. Certain 3 pointers, for example, are noticeably harder to solve than an average 4 pointer.
Secondly, a small handful of puzzles were poorly chosen. A few 3 pointers, for instance, have "natural-looking" (i.e. what an over-the-board defender would most likely play) 7 ply solutions, but also require you to calculate lines up to 14 ply if the defender plays optimally (not just via spite checks and such). These two solutions are often wildly divergent from each other. Granted, the first couple solution moves would be decisive even if they weren't mating, but for a book emphatically about forcing mate, Nunn should have replaced these examples with better ones. It leads to unnecessary frustration when you're expecting at most a 9 or 10 ply solution, realize you still haven't forced mate and after second guessing yourself and constantly checking/re-checking and finding nothing better (all while struggling to correctly visualize the position), you read the solution and find that, yes, you really were supposed to consider a line that was 14 ply.
Bottom line: this is a solid puzzle book and will almost certainly benefit those in the 1300-2000 elo rating range, possibly above. However, I can't truthfully opine this to be Nunn's best work of literature due to the above two fairly mild but still annoying flaws.
One last note: I am currently an unrated player, but would estimate my over-the-board elo rating to be in the 1500-1700 range given my blitz play against similarly rated players.