How can you develop great Active Directory Skills?

Any thing about Microsoft tech. you should begian with microsoft technet.

http://technet.microsoft.com/en-us/library/aa991542.aspx

read about three things not one .

1- LDAP.

2- Microsoft  infrastructure (DNS,WINS,DHCP).

3- Kerberos of KCC

 

by understanding the LDAP.
for more info about LDAP check bellow link http://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol