This book is for experienced Java programmers. I assume you know the Java language syntax and the Java API. I also assume you have strong object-oriented programming skills. These assumptions will allow you to quickly get into the essence of Scala and make use of it on real applications.
Developers who are familiar with other languages can use this book as well but will have to supplement it with good Java books.
Programmers who are somewhat familiar with Scala can use this book to learn some language features that they may not otherwise have had the opportunity to explore. Those already familiar with Scala can use this book for training fellow programmers in their organizations.