I was playing the new Total War:Age of Charlemagne and realized that I had only the slightest idea of who any of the major factions in the game were.
Ideally I'd prefer books that are more of an overview; mostly I want to know who the major powers were, what they did, and how it affected the later Middle Ages that I'm more familiar with.
Chris Wickham's The Inheritance of Rome: A History of Europe from 400 to 1000 (2009) does exactly that. Wickham is an excellent medievalist and his breadth of knowledge really shines in this book, as he discussed basically all the factions covered in the game. It was also written for the general public, so it's a very engaging read. In case you are interested in church history as well, Peter Brown's The Rise of Western Christendom: Triumph and Diversity, A.D. 200-1000 (2003/2013) is a great introduction to that topic and covers the same time period.