On a positive note, the book covers exactly what the title claims: the essentials. But, I was expecting something more (purchased online, sight unseen). To be fair, my criticism has more to do with Sun's pricing policy than at the book itself. Perhaps, I live in an archaic, bygone pricing age.
This is the book for the administrator who needs a concise reference on how to set up and administer ZFS and how to address common failures, with each (every?) feature described and illustrated in a logical sequence. It includes both the basic and some of the less-frequent tasks (migrating UFS->ZFS pools, patching ZFS boot environments, etc.)
It's not, however, a thorough treatment of ZFS. It doesn't cover ZFS internals and implementation nor does it provide a wealth of insight into diagnostics and recovery from disasters other than a high-level treatment of "snapshot" restores and "resilvering" (disappointingly, the screenshot of a "spool status mpool" showing a "degraded" pool in chapter 2.7 refers the reader to the Sun site for what to do and the accompanying text merely echoes that advice!)
Oddly, the book doesn't address the use of ZFS with Solaris 10 zones (containers). For that level of detail, one must refer to the ZFS Administration Guide on the Sun web site (no charge). That information wasn't deemed "essential". However, ZFS with Virtual Box as a lab (i.e., practice) environment is demonstrated.
So, do I recommend the book? Yes, especially if you can get someone else to pay for it (e.g., your company).