H.G. Wells is best known for his science fiction (e.g The Time Machine, War of the Worlds), but he also wrote a comprehensive history of the world around the time of WWI. I read this a few years ago and was really impressed with how he explains history in terms of themes (e.g nomads/settled peoples, priests/secular kings).

Even if you don't want to take the time to read the whole thing, this is a great reference for getting overviews of different periods of history. Although this book was hugely popular for many years, I believe it's out of print now, so you'll want to try an Amazon seller or maybe a local used bookstore. I got my copy at Powell's in Portland, which had a good selection when I was there.