Underscore's Scala books are now open source 9 years ago
As a sometime contributor to the slick project, I think the "Essential Slick" book, that is one of those that has been open-sourced here, does fill a gap in the documentation. I've recommended it in the past and now I can just link to it. Big thank you to Underscore for doing this.
It's an easy read with good examples that helps you get to grips with the concepts.