If you're interested in fiction I'm going to say, Anathem by Neal Stephenson It's not even my favourite Stephenson book (that would be Seveneves) but it's the

If you're interested in fiction I'm going to say, Anathem by Neal Stephenson It's not even my favourite Stephenson book (that would be Seveneves) but it's the one that made me reflect the most on philosophy and meaning.