I would talk to him and ask him more specific questions. Why did they think he molested his sister? If you still feel uneasy, maybe step back. You might need time to reflect on this and pause with him. The book thing is a little odd. Some people are odd people though. What grounds did the police have for thinking he molested his sister? Did he go to trial? Is there anything else about him that bothers you? Regardless, something feels off for you and it doesn't sound like you have all the information.