Here is a great book that helps in understanding the Windows Security Model, and I was pleasantly surprised to find it available online in it’s entirety…for free!
This is a great book to purchase as it is something you will refer to many times because of the depth of the knowledge. If you ever wondered more of what a SID, ACL, or Impersonation, this is a good source of information.
Another book in the series that I don’t think anyone interesting in Active Directory should be without even if you don’t program in .NET is the Directory Services Programming guide below. Even if you are not a .NET developer, the insight will help those who write in JAVA, and other languages understand the nuances of Active Directory which is more than just LDAP.
Both of these guides will help you understand Kerberos, Active Directory, ADLDS (ADAM), and hopefully assist with integrating other applications into your environment. The accompanying website, DirectoryPrograming.net, provides additional samples in C# and a great community for engaging around the technology.