i don't think reading other people's opinions will make you more opinionated. opinionated is a personality trait. opinionated has little to do with knowledge.
if your local library is anything like mine, you'll have very few options on the subject. i would literally just pick up anything that looks a little bit legal especially cuz it's free.
but if you really want a suggestion, my favourite is The Social Contract by Jean-Jacques Rousseau. or anything by John Rawls.
if you want to become more opinionated and learn stuff, i would suggest listening to podcasts. Freakonomics is a good one but really just fish around and listen to lots, especially Brexit related and British politics since you will be largely committing your life to the subject first year.