Excellent aside from the points cited below.
Gray does a masterful job of presenting a picture of the state of warfare at the end of the 20th century, then looking at all its likely permutations in the 21st. The title expresses his general feelings - he doesn't like war, he's tired of it, and he's equally tired of pronouncements that either the whole nature of war or the aspects of human nature that cause war will somehow change because we're in a new millennium.
He outlines the military history of the last three centuries and looks at all the factors that play into warfare, most of which aren't new, some of which are. He talks about conventional (uniformed-military on uniformed-military) war, guerrilla war, terrorism, cyberwarfare, and war in space, among other things, as well as doing a solid job of looking at the interplay of politics, economics, industries, ideologies, and warfare. He does this while making the book a lively and entertaining read, not always easy given these topics.
I have, however, a couple of quibbles. First, Gray is a true fanboy of Clausewitz and cites him over and over, sometimes when the applicability of Clausewitz's thought is dubious, and overlooks the fact that some of what Clausewitz wrote was just plain mistaken. For example, Clausewitz wrote that the goal of war has to be the destruction of the enemy's armed forces, and that hasn't been true for a long time if it ever was - which leads to my second objection.
Gray is fairly derisive about the term and concept '4th generation warfare', but his argument is a straw man - what he first sets up as the definition of 4th generation warfare, and then debunks, isn't the general meaning of the term at all. He uses the term to mean warfare between governments and non-state groups, i.e. most of guerrilla warfare. That's one means of 4th generation warfare, but not its end. To quickly sum up:
1st generation war was mass-oriented - whoever had the bigger army (or could bring the most to bear at the decisive spot on a battlefield) usually won. It was, as Clausewitz wrote, about destroying the enemy army.
2nd generation war was firepower-driven. Think World War I. Also true to Clausewitzian doctrine.
3rd generation war was maneuver-oriented - we can look back to Napoleon and to the U.S. civil war - think Nathan Bedford Forrest and his 'git there fustest with the mostest' - and really came into its own with mechanized warfare in World War II. It diverged from Clausewitz in that its goal was not necessarily to destroy the enemy armed forces, but to maneuver to a position where they couldn't win, so they'd usually give up.
4th generation warfare aims at the minds and will of the decision-makers who decide to begin and continue wars rather than aiming at the armed forces - in a dictatorship, the mind of the head of state; in a representative government, the executive, the legislature, and the voting public. 4th generation war came into its own with the development of electronic mass media.
So while, as Gray says, guerrilla war and terrorism aren't new, 4th generation warfare is a fairly recent phenomenon. For that reason only, I'm giving this 4 rather than 5 stars.
Still, a very well researched, organized, and thought-out book, and well worth reading.