Napoleon & Josephine: An Improbable Marriage (Evangeline Bruce, 1995) is a very readable dual biography, a real page-turner even though there's not exactly any suspense as to how it will turn out!
While the book focuses more on Josephine than Napoleon, it's such an intensely personal look at their lives that I feel like I understand him better than I have before, and I think he was the most selfishly ambitious person I've ever encountered. Of course, it takes a certain amount of selfish ambition to aspire to high political office, IMO, even among the saintliest of leaders. But for Napoleon, AFAICT there wasn't ANY leavening of patriotism, principle, or ideology--it was all about what brought him the most power. Any good or evil he committed was almost by accident. A strange, strange man.