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
Wanderlist: LDAP Resources - List of links to LDAP resources, ranked by most votes received by site visitors. - http://www.wanderlist.com/ldap/
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
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
What is LDAP? - Brief overview of LDAP, with links to further resources. - http://www.gracion.com/server/whatldap.html