In Introducing SQL Server 2008by Peter DeBetta (ISBN: 9780735625587), you’ll learn about major new features in SQL Server 2008, including security, administration, and performance. Downloading the book require registration.
This initial installment of the book is very small; it is devided into 11 chapters and about 33 PDF pages. I guess it is a good focused book on the new features of SQL Server 2008. Myself didn't read it yet.
I also recommend to visit SQL Server 2008 Learning Portalthere you'll find free e-learning offers such as Collection 6187: What's New in Microsoft SQL Server 2008 (includes three free clinics).