It starts with a VERY brief explanation of CSS selectors, inheritance, the box model, inline model, floating, positioning and the table model. Note that these don't substitute in any way the text in CSS, The Definitive Guide.
Then comes the CSS 1,2 and 2.1 properties reference, with the properties, values and their meaning. A very brief example is given for the property but it is only a snippet showing a selector using that property, nothing more. It is easy to search and, is small enough to be carried anywhere without causing much hassle.
It has the 10% that allows to do 90% of the work and is extremelly usefull to have around when writing Style Sheets.