Warfare, State and Society in the Byzantine World 565-1206 - John Haldon.

I had to read this in great depth for my dissertation. It was an informative and interesting book, but there are two fairly serious problems. First is that Haldon never uses a word where he can use a paragraph. Quite a few times I saw something hugely convoluted that he could have conveyed more clearly with far fewer words. Second is that the book's structure isn't very good. The chapters ostensibly deal with various different aspects and developments of the east Roman empire, but the nature of the book means that such divisions are highly arbitrary. It's probably for the best that he didn't try to enforce them, but on the other hand they seem redundant in the extreme, and it can be rather tricky to find what you want when it's scattered among any of several chapters and each fragment adds just a little more to the whole.

Also, don't go into it without a working knowledge of Byzantine history. Haldon gives plenty of historical examples and explains how events influenced Byzantium, but there's no general overview of how everything fits together.