Min is for '' en '' (for example en pojke = min pojke , en flicka = min flicka) and Mitt for '' ett '' (for example ett barn = mitt barn, ett äpple = mitt äpple) I figured it out thanks to Wikipedia (but in French) I guess you can search for better explanations if needed
We do that with things when you're 1) expected to have your own ones 2) you're expected to do whatever to your own ones rather than to someone else's. I always tell people that Jag borstar tänderna 'I'm brushing my teeth' is the perfect example. – Not everyone has teeth, but in principle everyone is more or less expected to have them. And it's most likely that you'll be brushing your own. With books, these assumptions just aren't strong enough, so you need to specify.