Although it's 30 years old, Colin Greenland's book remains the only significant study of a fascinating and important but largely overlooked period in SF history. On that basis alone, it's invaluable. However, it's not just unique, it's an incisive text that will increase any reader's understanding of some great but frequently difficult books.
The book starts with two chapters placing New Worlds and the British New Wave into a historical and cultural context, before discussing three key themes (sex, "anti-space" fiction, and inner space). This is followed by the book's heart - a chapter each on the key works (from the New Wave era) of Aldiss, Ballard, and Moorcock. This is some of the most illuminating SF criticism you'll ever encounter. The observations on Ballard are relatively familiar, probably because he became celebrated in "proper" literary circles, but the Aldiss chapter brilliantly highlights what an ambitious, clever, capable and humane writer he is, while the discussion of the Cornelius quartet is far clearer and more informative than most commentary on these fascinating but challenging novels. Greenland admires these writers but isn't blind to their faults. He's particularly wry in pointing out that the enduring popularity of the Cornelius Quartet is almost certainly due to what its author would consider the wrong reasons.
The concluding chapters tackle stylistic practice, stylistic theory and the wider place of concepts of entropy in contemporary (non-SF) fiction. These are more academic than the earlier chapters, but still full of rewarding insights. Greenland concludes that British New Wave SF is part of the continuum of, for want of a better phrase, post-modernist fiction, but also helped to create an environment that made such fictions very widely read. This rings true. You can look at contemporary SF and almost imagine that the British New Wave never happened, but if you then look at some of the more popular and artistically successful serious fiction of the last few decades, it's hard not to agree that New Worlds had some impact on creating an audience for these.
Other interesting topics covered en route include the differences between the British and American New Waves in SF (essentially, Greenland sees the former as being about a transformation of substance and the latter about a transformation of style, though it's not quite that simple), the profound debt the British New Wave owed to the Surrealists, some examples of Really Rotten New Wave Prose, reminders of some great lost stories, and the reasons why New Worlds was both entirely part of and utterly distinct from the cultural changes of the 1960s. It's also, for an academic work, highly accessible to a non-academic reader. The only problem I have with the book is the typesetting: quoted texts are presented in the same font as Greenland's own prose, and only slightly indented, which makes it slightly harder to follow than it should be. Other than that, it's unconditionally recommended to anyone with a serious interest in the British New Wave or its key authors.
PS An earlier reviewer felt this book was dated due to advances in critical and post-modernist theory since the original publication date. This is almost certainly true, but - other than the huge sigh of relief that will prompt from most sensible readers - does it matter? Greenland certainly had some awareness of these issues, particularly in relating aspects of Ballard to aspects of Barthes, and he's also aware of Delany's structuralist critiques of Disch (and Moorcock's response, which was to describe Delany as "functionally illiterate"), though he only really mentions it in passing. More recent academic discussion of SF which takes a post-structuralist/post-modernist line has no interest in British New Wave SF in any case. Either it doesn't follow the party line of having an obvious agenda to deconstruct (for these writers, the canon only really consists of Le Guin, Russ, Delany and Cyberpunk), or it's opaque to that particular line of analysis. And even if it were possible (which I doubt) what would we gain from a post-modernist reading of "The Atrocity Exhibition" anyhow?