I've had some luck with 'Beginning SQL Server 2005 Administration' (Dan Wood, Chris Leiter, Paul Turley - Wrox.com).

Whether you devote additional time to 2000 specific books probably depends on what you will mostly work with - 2000 is going on extended support soon (if not already, I don't recall the date), so many organizations are trying to upgrade 2000 to 2005 whenever possible.

I'm very new to being a DBA myself, and support both 2000 and 2005, but I haven't yet run into any issues that required me to read a 2000 book specifically (knock on wood).