The Definitive Guide to Yii is the most comprehensive Yii documentation that gives the definitive description of every feature of Yii. It is being constantly updated to reflect the latest enhancements and changes. A PDF version of this tutorial can be found in the Yii releases.
Larry Ullman's Learning Yii Series is best suited for people who just start with MVC and/or Yii programming. (Difficulty level: 1)
The Yii Blog Tutorial describes in detail how to use Yii to develop a practical Web application. It explains the usage of Yii in a systematic way. You may also find a PDF version of this tutorial in the Yii releases. (Difficulty level: 3)