I agree that practice is the best method of advancing your CSS skills, however, designers can benefit from some pointers, providing they are ‘real-world’ techniques supported by ‘real-world’ browsers. Books like CSS Mastery are good, taking your basic grasp of CSS and leading you down the ‘advanced’ path rather than books like Pro CSS Techniques talking of advanced CSS capabilities that aren’t supported by IE (despite the hate for it, still the most used browser!)