The good folks over at Microsoft Press are offering a free e-book entitled "First Look: Microsoft 2010". This book is designed to give you a 3-part look at the inner workings of the upcoming Office 2010 and how it could enhance your productivity. The entire book is in PDF form and is roughly 10.5 MB in size.
To download the book, click here.
If you wish to download the Office 2010 Public Beta, click here.