LDAP

IBM Redbook: Understanding LDAP
Basic principles and concepts of LDAP. Minimum product-related or vendor-specific information.
http://www.redbooks.ibm.com/abstracts/sg244986.html

IBM Redbook: Using LDAP for Directory Integration
Explains directory and LDAP concepts and tasks. Contains examples based on IBM SecureWay, Lotus Domino and Microsoft Active Directory.
http://www.redbooks.ibm.com/abstracts/sg246163.html

Wanderlist: LDAP Resources
List of links to LDAP resources, ranked by most votes received by site visitors.
http://www.wanderlist.com/ldap/

What is LDAP?
Brief overview of LDAP, with links to further resources.
http://www.gracion.com/server/whatldap.html

Wikipedia: Lightweight Directory Access Protocol
Encyclopedia article about the protocol, including origins and influences, terminology, and technical information.
http://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol