The length certainly is intimidating, but every page is packed with information. A lot of so-called "security" books are full of fluff and recycled advice that is freely available on the web. Anderson's book is a well-thought-out encyclopedia of information that you will need to design dependable, resilient, secure systems.
As an author, I can tell you that it is easy to tell the difference between a book that someone has rushed to press and one that is a carefully constructed reference. Anderson took the time to do his book right.
Take the time to read the book; don't hurry through it. Then come back to the book again in a year--I can guarantee that you will learn as much the second time through as you did the first.