My springboard Hesse book as an adolescent was Demian - and then I read all the rest too.  

At the time, my mother told me that my father would have loved those books too when he was young (it wasn't quite her thing), and at the time I thought I'd still love them at his age.  However, she had a point - years later, I re-read Demian which had so enraptured me as a teen, but that same sense of magical epiphany was no longer there.  

A great writer though, and an unabashed seeker after spiritual truth.