I'm reading SQL Server 2008 Internals at the moment. I say 'reading', because I think it's going to be long-term thing. It's just so full of useful information, that I'm sure I'll be reading it over and over for a long time yet.
Kalen Delaney's books are always great, but in this one she has help from Paul Randal & Kimberly Tripp, Conor Cunningham and Adam Machanic — all SQL Server legends in their own right. The book they have made is just excellent, and should be read by everyone who wants to get deeper into SQL Server. There are some sections I've only skimmed over so far, whilst others I've read thoroughly. In time I think I will have read every page multiple times, but this is definitely a resource that can be read that way (yes, Kimberly, it has a good index).
It covers so much useful stuff it's hard to think of a better resource for SQL Server 2008. It doesn't go into design very much, but it will affect your design decisions. It doesn't go into writing queries, but it will affect the queries you write. I really think this book would be an asset to anyone who wants to know more about SQL Server.
[Updated: This link should take you to where you can find and buy the book. MSPress have 50% off their books until June 30, 2009, which makes this book an even better investment]