Most Xen documentation on the Internet can be a tad focused on the single-computer, single-admin personal-use xen administration case. This book, thankfully, is not.This is definitely the book to keep on your shelf if you require tips and tricks for setting up your own VPS hosting service, with its world full of malevolent users needing to be kept in their place, quotas for bandwidth, disk I/O, CPU time, and memory usage, and allowing your users to configure their own instances without you having to step in every time they blow out their /boot partitions.
There are plenty of concepts covered in here for other use-cases (besides just hosting your own VPS provider) as well, including remote-mounting disks over NFS/iSCSI/AoE, migrating live Xen instances across a cluster of servers, and backing up disk images and machine states.
The Book of Xen provides a fair and balanced view of Xen management; that is to say, while it it does talk often about the many distro-specific ways of easily bootstrapping and configuring a new virtual server (like Debian's debootsrap, Red Hat's virt-install, or even creating images in Citrix XenServer) it also covers vendor and distro-neutral ways of performing all the required installation and management tasks. The Book of Xen is also fair in that it also goes on to describe the use and configuration of Microsoft, BSD, and Solaris Xen dom0 and domUs as well, with the caveat that support for Xen is weak and upcoming on such platforms as FreeBSD, and that HVM is required for many of these more exotic operating systems like "Microsoft Windows", as there are no Xen hooks in the Windows kernel.
I particularly liked the Book of Xen's first chapter, which, unrelated to the rest of the book's sysadmin-oriented content, was a good overview of the technical underpinnings of the Xen hypervisor platform, and how it interacts with the hardware and virtualized machines from a very low-level perspective. As it is stated later in the book, and something that I agree with, the authors believe that one must know a technology, how it works, and its more basic manual and command line tools, before ever trusting a GUI or web interface to do the same. It will also surely aid debugging later when something goes wrong, as the administrator will have a good idea as to where the problem might lie.
All in all, I liked the book and would recommend it to anyone setting up their own Xen servers, however, I wished it would have had more information about Xen on the Intel Itanium (which is touched upon in the book as being a supported platform, but not talked about further) and I wish it had talked more about some of the topics they covered, like giving users access to their own xen management consoles, in the common situation where there are many physical machines that a user's instance could be on, a situation which completely broke their offered solutions for this situation and others.