The "Beginning" book is for a reader who may not have any previous programming experience. You'll learn various programming concepts as you learn ASP.NET 3.5. It also follows building a single example start to finish to illustrate the main points.
The "Professional" book is for a reader assumed to be an experienced programmer. While it doesn't completely skip into level ASP.NET 3.5, it blows through it quickly. It doesn't assume previous ASP.NET experience although that will help. You will need to understand some object oriented language to use it. It's also more comprehensive and goes deeper on the later, more advanced topics.
Hope this helps.
Jim Minatel
Acquisitions Director
Wiley Technology Publishing
WROX Press
Blog:
http://wroxblog.typepad.com/
Wrox online library:
http://wrox.books24x7.com