This book esplains the Linux kernel internals. It is a mix between a book about Operating Systems, and a book about Linux internals. Every part is initially explained from a theoretical point of view, then the book shows how that part is implemented in the Linux kernel.
If you want to start hacking the kernel, this may not be the right book to start with. At least, you should read also "Linux Device Drivers" to have a more complete vision about how to make Linux do what you want.
Instead, if you want to understand how things work inside Linux, and how start collaborating with the kernel community, then this is the right book.
Unfortunately, the mechanisms inside Linux change very quickly. For this reason, this version (i.e., second) of the book might be too old. For instance, it does not contain the latest (i.e., CFS) Linux scheduler. You should buy the third edition, which, in my point of view, is a minor update of this book, adding some information about the new CFS scheduler and about some new synchronization mechanisms.