Is this book yours?

Yesterday, my wife and I were cleaning the smaller room, I put my books in this room. I often read books. But when she took out a book and showed to me, I asked her, "Is this book yours?"

That was laughable. In fact the book is mine. 4 monthes ago, I bought 10 books for one time. But there were some of them that I never read because I was busy, so when she showed it to me, I couldn't know this book at once and I reckoned it was hers.

So I joked with her and said, "You have an unassiduous husband."