I've heard the Accidental DBA is a very good book and frequently mentioned on these forums, Kalen Delaney's SQL Server 2008 Internals is another good book, and worth having ont he bookshelf.

I'd also recomend SQL Server Execution Plans by Grant Fritchey, if you dont have a copy, as it will give you an understanding of how to disect query plans and suggest/make query changes.

There are also a couple of good Internals books by Itzik Ben-Gan, that while they are more aimed a developers they also have some very good nuggets of information for DBA's.

Read MS white papers around Lock contention and High performance DB's, as they often have best practices and some very good ideas.

In the end doing the job on a daily basis is the best way to learn, the books and white papers are mainly reference materials and handy goto guides.