There are three things that you should consider before buying this book. Firstly, are you a general reader or do you have a genuine interest in religion and politics? Secondly, do you prefer "neutral" commentary, or to read through the lens of the author? Finally, do you have a better than average background knowledge of C20th politics? The answer to these questions will largely determine whether you buy this book in the first place, as well as whether or not you manage to finish it.
Concerning the first question, the book interweaves politics and religion to the point where only a genuine interest in both would be enough to sustain the reader's interest. The subject matter is "heavy" anyway, and this coupled with the book's length mean that only the most interested need apply. Concerning the second question, this is most certainly not neutral, with Burleigh's views abundantly clear throughout, no more so than in his chapter on Northern Ireland, with his views expressed with somewhat unnerving ferocity. Having said that, his frankness throughout is a refreshing change to many history books, with their historians desperately trying to walk a neutral tight-rope so as to be all things to all readers, but not Burleigh. As a result, the heavy subject matter is much more digestible, punctuated with his frank statements that can at time genuinely amuse.
Concerning the third question, Burleigh's vocabulary is aimed at a highly educated audience, and he writes in such a way that assumes the reader has a very good background knowledge of the C20th and its political history. For instance, when talking of Poland he makes reference to the Home Army and the ZOMO, in both cases not offering a definition of what they were, on the assumption the reader will have sufficient knowledge about what they are already (or assumes that at very least they will be sufficiently motivated to find out).
One part of the book that is interesting to highlight is Burleigh's detailed and spirited defence of Pope Pius XII, and he conclusively rehabilitates Pius' ill deserved reputation as "Hitler's Pope". Alas, this section of the book is too detailed, to the point one could be forgiven for thinking this rehabilitation is a personal crusade on Burleigh's part. Indeed, the whole chapter is punctuated by long, at times quite droning quotes and vignettes about what the Catholic Bishop of this German province said or what a Protestant Bishop said in that province etc.
Another point to bring up is one or two obvious conflicts that Burleigh could have made more reference to, most notably the Yugoslav war where religious identity played a major role in the conflict and its political settlement, yet he makes next to no reference to it. Instead he focuses on Nazism and Communism, and as such often regurgitates that which is already widely known about their evils. In this respect, he almost falls into the trap of doing what he ironically criticises other historians of doing earlier in his book, namely going over old ground on topics that are already widely known. Whilst as an award winning historical author he is better qualified than most to criticise this, he should also be better qualified than most to avoid it.
All in all, this book is a mammoth text, and I liked much, much more than I disliked. It covers the two deepest and most complex topics possible (religion and politics) and attempts for the first time to weave them together in an encyclopaedic account of their inter relationship. Nevertheless, the book has a few weaknesses: whilst Burleigh's undisguised views are at times genuinely refreshing, at times they are stated too frequently (as with Pius XII) and at times too fiercely (as with Northern Ireland). Also, he overlooks certain conflicts (e.g. the Yugoslav war) and certain social developments like the rise of the Christian Right in American politics in the 1980. In summary, the book is a powerful and serious attempt to bridge a gaping hole in the historical literature about the inter relationship between politics/religion the C20th, one that will be rarely attempted again, and thus for the foreseeable future the book will perhaps be the definitive text on the matter.