Patterns and Practices: A Guide to Claims-Based Identity and Access Control - Free ebook.
While I usually like conversation-style technical books (think: Manning's In Action series and many Wrox books), Microsoft's Patterns and Practices series is one of my favorite even though they feel so formal. Recently, my buddy Buck Woody posted a link to the freely available Patterns and Practices book for A Guide to Claims-Based Identity and Access Control, Second Edition.
It's the same book that can be purchased at Amazon.com for 23 dollars (make you holler.)