During my teenage years I bought: A Bible, A Qur'an, Zen and the Art of Shooting Bow, The Red Book, The Capital and The Communist Party Manisfesto by Karl Marx, Mein Kampf among others... They are still lost among other like-themed books at my Parent's house, probly in the same box as my old Playboys and Harry Potter books.
Having the books just meant that at some point he wanted to know wtf that was all about, however if his believes are based or if he finds some sort of guidance or comfort in the book... perhaps you should... Idk... step slowly away...
Fast way to solve it would be too go to your father and ask.
"So... I found this book here... any funny story behind this or should I be scared?"
Something like that.
Edit Note: I think people should really read most books that generated some kind of ideology, its a great way to understand and be able to criticize or defend what you believe in.