This is going to have to be brief for the moment - but I'm open to suggestions for new links!
The java jndi library (now shipped as a standard part of java 1.4) provides a good interface to directories. Unfortunately it is not ldap specific, which makes it rather more complicated to use. If you're willing to accept the overhead though, it has an excellently written comprehensive introduction at: Sun's JNDI Tutorial.
Open Ldap (Java)
There are also some java resources available (which I haven't evaluated yet) at openldap's - Java Ldap pages.