What level of Seam knowledge is suppose to have the reader and how deep goes the book?
A basic knowledge of Java web application development is necessary. The book starts from a very basic level and gradually dives deeper into the details of each Seam concept. For example, Part II: "Stateful Applications Made Easy" starts by introducing you to web development with a stateful framework. It then walks you through how stateful components can be used to develop an application. Conversations, which are at the core of Seam's stateful model, are then introduced and the book gradually dives deeper into the details of the conversation model. By the end of the part the chapters will have walked you through the conversation model, the conversation API, concurrent workspaces, nested conversations, web transactions and persistence context management.
Does contain performance best practices?
The book has an entire chapter dedicated to performance tuning and clustering: Chapter 30. Seam's multi-layer caching is also covered in Chapter 32 which can really help your application scale.
One advantage the book does have is the breadth of integrating technologies that are covered. This helps a reader understand exactly what it means when you hear that Seam is an integration framework and really what that buys you. It becomes simple to develop very complex enterprise applications that include PDF generation, email, timer jobs, complex security, bookmarkable (RESTful) web pages, workflow, business rules, and so much more!
As far as being better than other Seam books, I will have to let those who read the book judge that ;)