This book is a study on the politics of the Labour Party; specifically, it examines Labour's ideas, values, practices and policies concerning the Party's brand of "socialism". Labour's "socialism" is exposed as having little or nothing to do with the transformation of capitalist society in a genuine socialist direction - rather, for Labour "socialism" means only the inclusion of representatives of the working class (alongside representatives of other classes) participating in the parliamentary process.
Yes, Labour's "socialism" is shown to encompass social reforms - but, for Labour, the purpose of socialist ideas and practices is merely to reform the existing order of things, not to bring about any sort of radical alternative. This type of reformism differs from the objectives of many other European social democratic parties founded between 1880 and 1914, which considered such reforms as a means to bring about a revolutionary transition. This form of European social democracy was distinct from both (1) the revisionist politics of Labour, and (2) the insurrectionary strategies of the Bolsheviks.
Miliband provides an excellent discussion on the historical development of Labour - from its formation in the late 19th century, based on certain reformist and revisionist ideals and ethics, through to its advance as a leading political party in Britain by the mid 20th century. This discussion is comprehensive, well referenced, and will still serve as a useful tool for students of the history of early to mid 20th century British politics and the labour movement.
Miliband found that the purpose of narrow reform and revision, in and of itself, is inherently conservative - and he defined this brand of limited socialism as "Labourism" - distinct from proper, genuine socialism. Given the dominance of Labour as a representative of the working class, Miliband concluded that the interests of this class - the building of socialism - cannot be arrived at through Labour. Indeed, there is an implicit argument made that socialism cannot be achieved through parliamentary means - although nowhere is this view explicitly advanced, and elsewhere Miliband accepts the use of parliamentary strategies in the advance of socialist goals.
First published in 1964, this book became a "standard" on the Left for its understanding of Labour. Miliband subsequently wrote a useful postscript, updating his discussion so as to encompass the 1960's - when, once again, Labour was in office. I repeat, anyone interested in the Labour Party or British politics during this period should read this book.
For a more recent study, and one that in many ways can be seen as bringing Miliband's analysis fully up to date, see "The End of Parliamentary Socialism" by Panitch and Leys (2001).