So just to be clear: this isn't "a book" we're talking about.
I wouldn't start here. My primary reason for digging in is it's a grand survey across a broad period to fill in the holes that I normally would not focus on. Without this book I wouldn't focus on the "Dark Ages" for example and they're actually one of the most interesting periods. Of course, without this book I wouldn't focus on 18th century france and the fuckers spend about 3,000 pages on them.
Really, if I were to recommend one history book, based on what you're telling me, I'd go with Tony Judt's Postwar.