This book was first published 3/17/2006, since then, virtualization has evolved, new versions were released and new technologies have emereged. The book talks about VMware GSX which was discontinued over a year ago, ESX 2.5 (Current version is 3.5 with 4.0 around the corner) and MS Virtual Server which is being replaced with Hyper-V on Windows Server 2008. No mention of any opensource option (Like Xen) which wasn't mainstream in '06 yet. A good read if you're interested in the history of virtualization.
