Well, I would say that whether it's odd or not probably depends on what your father does for a living. If he's a historian, or involved in some other sort of scholastic work where having some knowledge of the book might come in handy, then it might make sense. I've even heard of people reading it in certain college history courses, so if he went to college then it's possible that it was a book he had for one of his classes. Or, maybe you have German heritage, and the book belonged to one of your ancestors? There are lots of reasons for which one could own it without being an adherent to it. If I were you, I'd ask him why he has it. Unless he's an unreasonable man, then I'm sure there'll be a good innocent explanation.