Definitely, a classic!
This is simply the most quoted International Relations Theory book in the US or in the UK. Waltz begins by an exposition of his own views on what theories are and what are the requirements of sound theorising in international politics. He, then, proceeds to a brief evaluation of previous theories of international politics based on these criteria. He concludes that a theory of international politics must be systemic in order to account for structural causes of international political phenomena. He provides a slightly Durkheimian explanation of the notions of system and structure and then he moves to an identification of the main features of the international system.
A system encompasses, besides interacting units (the key ones are sovereign states), a structure. The structure of a political system includes three main features that need to be considered in order to account for systemic outcomes: the ordering principle, the level of functional differentiation of the units and the distribution of capabilities among them. The ordering principle of the international system is anarchy because there is no central authority. This is unlikely to change in international politics. Anarchy drives states to self-help behaviour, which, in turn, drives states to be as self-sufficient as they can in order to assure their own survival as units of the system. Thus, functional differentiation among relevant actors (the great powers) is considerably low. They do not specialise in anything, because this would involve depending on others for the non-specialised agenda. Because anarchy is unlikely to change, functional differentiation that results from self-help behaviour is also unlikely to change. Therefore, variables in the international system, if any, are to be found in the distribution of capabilities (aka., power) among states. That is, specific configurations of this distribution yield specific outcomes.
Central as it is, the distribution of capabilities becomes Waltz's key category to analise international political outcomes. He argues that multipolar systems (in which there is a more equal distribution of power among states and, therefore, a greater quantity of relevant actors) tend to instability (defined as war among at least some of the most relevant actors). For example, alliances are always shifting in such systems in order to preserve the balance of power. Bipolar systems, on the other hand, are seen as the most stable (because there are only two great powers, it becomes obvious for one that the other is the rival to be balanced by internal efforts).
Waltz's book has a generally acknowledged merit of founding a neorealist research programme, defending a methodology for the study of international politics that resembles neoclassical economics and, more importantly, being a key target of criticism from all other streams of thought within the discipline.
Every student of political theory and international politics should definitely read this book. Buying it is another issue. To begin with, its price is too high. Moreover, one's sight becomes tired just looking at the font that was used in the printing. Lastly, there is also a rumour about the possibility of a brand new edition coming out soon which will include other writings by the author.