I grabbed Pro SQL Server 2005 High Availability by Allan Hirt recently to add to the library and I've been pleased with it even though it's expensive as books go, close to $60. Really good coverage of everything you need to know to figure our your HA strategy, with lots of emphasis on clustering (but all types of HA are covered well). The only part I wish had more was on hardware related stuff - split mirror/business continuity volume stuff. As he mentions each vendor is different, but a walk through of any one vendor might make it easier for readers to better understand their options.

Highly recommended.