Pragmatic Guide to Sass, by Hampton Catlin and Michael Lintorn Catlin
When I was first becoming a professional programmer, I worked with a consultant who, despite being newer to the language and IDE I was using, was able to teach me some new syntax and features. When I asked how he knew these things, he pulled out an intermediate book on the language and commented, "You find a lot of new tricks reading the manual cover to cover."
In that vein, I picked up the Pragmatic Bookshelf's Pragmatic Guide to Sass by Hampton Catlin and Michael Lintorn Catlin. It is a relatively short read at 126 pages, made shorter by the function reference that starts at page 107, and the Haml introduction at page 114
The book is a fast read, made up of tasks and Sass solutions. The basics of the concepts are introduced: variables, imports, mixin, extend, math operators, @each and @if, along with a number of conventions that would surprise anyone who didn't know about them. I'm thinking of the underscore-prefixed file name convention for Sass files that are not standalone after compiling (partial imports) with that comment.
The book is a couple years out of date, and could be updated to include new Sass and Compass features such as namespacing and possibly defining custome sass functions.
As a introduction book, which this book describes itself as, it works. As a refresher book, which this book describes itself as, it works less well. It skims over some of the whys one would use a feature, in favor of how one uses a feature, and doesn't contrast features much. As an introduction, this could be okay for most designers or developers. I prefer the whys, even if only in a sidebar.