If you're a web design novice, this book gives you strong guidelines but isn't gonna tell why your floats don't work properly in Internet Explorer.
Already a web-savvy Digital Creative? This book is a hipified reference manual full of relevant, fun (and sometimes irreverent) reminders and inspiration. It's biggest strength, however, is its distillation. If you already have the skills and mind to create the next coolest site, you may just need a peg to hang an idea on. A 'manual' like Guidelines for Online Success offers that peg without all the distracting dressings that come along with web-trolling for inspiration.
If your major design decision boils down to vertical vs. horizontal navigation (or radically both!), then you don't need this book. You're good to go.