As I've mentioned in the past, I'm currently working on an Exchange 2007 roll-out and it's on Windows 2008 servers.
During this endeavor, I've been using the Windows Server 2008 Inside Out book as a reference guide and I must say it is quite good. William Stanek is a good author and the book delves into many real work scenarios and holds a lot of good IT on-the-job kind of advice. And at almost 1500 pages, thankfully it's an MS Press book, which means it comes with a searchable PDF eBook version.
The book is written for the Intermediate/Advanced reader which is something to keep in mind. I definately recommend it.