This book was somewhat dated at the time of its original release and is now way behind the times and in need of a major rewrite.
The description of the difference between null and undefined is confusing. I had to read it several times before realising that the author had in fact got the two the right way around. On first reading it gave the impression that the definitions were backwards.
With the chapter on the Object object, the book starts to become more useful although the omission of the new ways of creating objects introduced in ECMAScript 5 means that the chapter is very incomplete. It doesn't even cover the old equivalents to these new methods.
The book becomes even more useful once you get to the chapters on the Function object and the following chapters on 'this', closures, and prototype. The information presented in these chapters is really useful and well laid out.