[Haskell-cafe] References on GADT's

Hi, I'm climbing up the standardized ladder of functional programming: https://news.ycombinator.com/item?id=13067741 The next topic on my list is to gain some understanding and practice on GADT's (competent section of the ladder). Do you have any resources that you would recommend? Are there any books that cover the topic? Thanks in advance, Damian.