After completing the first two courses of the Scala certification on Coursera I decided to go on with the famous book by Paul Chiusono and Rúnar Bjarnason to get a deeper understanding of all the underlying stuff. Now that I am two thirds into the book a lot of stuff feels more comfortable.
By now I am in chapter 12 and from chapter 10 on all the higher kinded types stuff, Functor, Monad, Applicative started: Really interesting.
Not all exercises are done right now, but I am in the making. After all I think it is really important (if not essential) to work through the book instead of just reading it. I actually started that way, but there was a point were I decided to go back to the start and read it again while solving the exercises and it is a lot more valuable... at least for me.