This is quite simply the best book I've ever seen on advanced Mac OS X system administration. If you think you know it all, this book will prove you wrong. It goes both deeper and wider than others, covering common tasks such as client management and Active Directory integration in extreme detail, along with a broad range of topics including Xsan and iSCSI storage, mail and calendaring services, third party deployment tools, iPhone deployment, and even Windows deployment through BootCamp and VMware.
While the ubiquitous Apple Training Series books are well written and better edited than this, they're primarily a study aid for certification courses -- providing step-by-step exercises but little depth beyond what's published in the Mac OS X Server documentation. This is the book to reach for when you've passed the exams and are ready to go to work.