Microsoft Protocols Technical Documents

MSDN has a library of technical documents called the Windows Protocols documentation set. Of particular interest are those that pertain to Active Directory schema classes and attributes, Active Directory Technical Specification, Active Directory Web Services, Active Directory Federation Services, Lightweight Directory Services etc. These documents have a lot of detailed information. They can be downloaded as PDFs individually or as a complete archive zip file. They are available at Windows Protocols, click on Windows Technical Specifications.