I highly recommend subscribing to the SQLServerCentral.com lists. Just skim the e-mail for anything new you want to learn about. If it has nothing, delete it. If it's something interested, either save it to read later, or read into it then!

Your Microsoft white papers are always going to be a great source to learn from. However, coupling the reads with sheer experience and hands-on "lab tests" are going to really teach you.

I am currently reading SQL Server 2008 Query Performance Tuning Distilled (Expert's Voice in SQL Server) by Grant Fritchney. It's a pretty good read in regards to server administration for database servers. I would highly recommend watching his webcast at http://www.sqlservercentral.com/Training/ first though. It's titled #6: Gathering and Interpreting Server Metrics. This will give you an idea of what to take/not take from his book.