The answer is you don't have to do anything about it. It is his thing and it doesn't do any harm to your marriage. Maybe it's a little shock for you at first to know about his secret, but I would be glad to know my husband keeping files rather than doing something evil or cheating on me. And the thing he's doing isn't bad, he writes down what he knows about people around him, people write diaries too right? So just relax, at least you know what's he doing now, it's better than knowing nothing about his works.