This is, quite simply, one of the finest examinations of the capitalist state ever advanced. Nicos Poulantzas develops an original Marxist approach which conceptualises the state as a 'social relation' (similar, in this regard, to Marx's treatment of capital as a social relation).
This project is, like much of Poulantzas' work, highly theoretical - the point, however, is that until this publication an adequate theorisation of the state was altogether lacking. Here, the state is treated as a 'strategic field' - a site at which various social classes, organised as social forces, exert their interests as part of the class struggles that take place in a capitalist social formation.
This book was originally published in 1978 - and, since then, some really excellent reviews have appeared elsewhere (I recommend these, as an adequate review of the book cannot be provided in such a short space here). Suffice it is to say, for anyone interested in understanding the theory of the capitalist state, this book is a crucial point of research and reference.
As far as I'm concerned, "State, Power, Socialism" is the most important work on the capitalist state ever written. Poulantzas' state theory develops from his political theory, as outlined in detail in his earlier publication "Political Power and Social Classes". This political theory proposes a 'regional' approach to understanding the different levels (economic, political, ideological, etc.) in a social formation. Put briefly, I consider his later 'relational' approach as not distinct from, but being an extension of, his earlier 'regional' theorisation. Regional theory allows us to understand the political; relational theory allows us to understand the state as a strategic site within the political region. As such, the two works complement each other - and I fully recommend both.