Hi,
you might want to check
http://download.microsoft.com/.../windows_server_2008_r2_e-book.pdf
The ebook covers what is new with SQL Server 2008 R2 - and one chapter is talking about Active Directory.
I don't know any other free eBook that covers Active Directory in english (In german I would even point to the openbooks of galileo computing. They are quite nice and offer free ebooks on multiple topics.)
With kind regards,
Konrad