For VMware customers still running ESX Server 2.5.x there is a very good news: the bestseller 450-pages book VMware ESX Server 2.5 Advanced Technical Guide, authored by Ron Oglesby (CTO at RapidApp) and Scott Herold (Director of R&D at vizioncore), is now provided for free.
Authors decided to release it for free now that they are about to release a new one on ESX Server 3.x.
Download it here.
If you still want a hardcover you may want to order it on virtualization.info Bookstore.