Ask HN: Learning about Type Systems

Hi,

I've been a Python / Javascript programmer for a long time and just last week started learning OCaml after listening to a podcast. OCaml has been great and I'm really loving the strong typing and bugs the compiler spews out before runtime. On HN I've repeatedly read people debating about Type systems and comparing them in different languages. I would love to advance my knowledge on the area! Are there any beginner books on Type Systems that helped you in the grokking the subject?

Thanks!