I would like to learn more about the LDAP, but a lot of the book I found on amazon.com are more than 2-3 years old.  I am afraid they're out of date.

Is there any good books that you guys would like to suggest?  I am interested in in-depth LDAP explaination/usages/design/schema design.
I don't really want to be product specific because the last 2 years, we already switched 3 directory server.  From Sun DS to OpenLDAP to Fedora DS.

Thanks in advance!