My name's Mark Craig. I used to work for Sun Microsystems, leading the Directory Services documentation effort.
In mid 2007, Sun donated code and documentation for version 6 of the LDAP C SDK. The doc also covers the LDAP Java SDK. In February 2011, I wikified the DocBook SGML version as the Mozilla LDAP SDK Programmer's Guide.
Older versions of that documentation are at http://www.mozilla.org/directory/csdk-docs/ for the LDAP C SDK doc, and perhaps somewhere on the Oracle doc site for the LDAP Java SDK.