The Scala language first crossed my consciousness over a year ago, at a stage where I was just starting to get into functional languages. But even with the various quick tour documents on the language web site, it was clear there were a lot more subtleties to this one that were not being explained.
With the stairway book, that gap has been bridged.
The book is aimed at the experienced programmer in 'C' derived imperative languages, with at least some familiarity with the Java™ language, and ideally some notion about functional programming techniques -- it is not by any stretch of the imagination a "my first programming book". For the intended audience, it is an extremely effective step-by-step guide to the features, and the syntax, of the language (this is a great contrast with e.g. Foundations of F#, which expends very little effort towards separating the accidents of the particular example from the generic syntax). While I am by no means yet fluent in the language, I feel that when I'm using it for hobby coding that I'm not just groping in the dark, but instead have a solid guide and reference to lead me.