Packt recently sent me a copy of VirtualBox 3.1 Beginner’s Guide to check out. 
At first glance the book is very entry level. The discuss setting it up on both Windows 7 and Ubuntu Linux. They also go into installing guest additions and sharing folders.
But digging deeper into the index they also cover more advanced features like running seamless windows and 3-D hardware acceleration. Later in the book they discuss storing data on virtual hard drives and also go into network configurations quite a bit.
The book seems well laid out. Each chapter introduces you to a concept then they get into details. The author then offers up a simple pop quiz to test your knowledge and then he summarizes the information covered in the chapter.
I will post a more comprehensive review once I’ve finished reading it.