Free MS e-book - a guide to claims-based identity and access control

Although based on earlier versions of SharePoint and Windows, this book provides very detailed explanation on all concepts of Microsoft implementation of claims-based authentication and access controls.

http://msdn.microsoft.com/en-us/library/ff423674.aspx

A paper version is available on Amazon:

http://www.amazon.com/Guide-Claims-Based-Identity-Access-Control/dp/1621140024/