The Authors of Sass and Compass in Action cover almost all of the capabilities Sass and Compass offers, while also including further information on topics such as contributing to the Sass/Compass community.
To gain a deeper understanding of Sass, an advanced course is offered by TreeHouse. They also offer a Compass Basics course (Compass is a Sass-based framework).
It's not uncommon for designers to be skeptical of Sass at first. The author himself was reluctant convert. "Sass for Web Designers" does an amazing job of explaining the "why" behind Sass and how it can make the life of a designer much easier.
The author, Dan Cederholm, explains the basics of Sass - mixins, variables, common functionality - in an easy to understand way and with clear examples.
"Sass For Web Designers" covers only the basics of Sass (such as mixins, variables and nesting), which for many designers may not justify the cost of the book.