This is an excellent, thorough and comprehensive investigation of ethnic conflict in the broadest sense, from ethnically-based democratic party politics to military coups, ethnic riots, irredentism and secessionism. In its almost 700 pages it covers every imaginable facet of the issue in considerable detail. To my mind, the work's greatest strength is the fact that rather than just presenting dry theory, multiple real-world examples are given for every point that's made. This, along with Horowitz's clear and engaging prose, means that even someone quite new to this topic can easily grasp the arguments. What's more, it means that as well as learning about the general theory of ethnic conflict, the reader will also learn a great deal about the history and politics of various countries around the world (chiefly in Asia, Africa and the Caribbean).
Some reviewers have suggested that the work is overlong, but I really can't say that any of the content feels superfluous at all. I admit reading through cover to cover was a slog that I'm unlikely to repeat (it took me three and a half weeks!), but I have no doubt I'll refer back to it in the future as a reference work.
The work's obvious apparent shortcoming is the fact that it was published in 1985 and so contains nothing about the ethnic conflicts that occurred since then, most notably those that have taken place in the formerly socialist countries of Eurasia since the start of the 1990s. However, it seems to me that much of the theory that Horowitz presents can adequately be applied to these cases. The limitation to this is the fact that the vast majority of Horowitz's arguments apply primarily or only to postcolonial, developing countries, and as such may be more relevant to the countries of the Caucasus and Central Asia than to those of Eastern and Central Europe.
My only real criticism – and it is not a huge one – is that Horowitz sometimes gives too little context for his examples. For me, this was largely an issue with African examples, about which I previously knew little. Fortunately, reading in the 21st century it's no big deal to quickly get on Wikipedia to find the background information you need.