Probably the books that were written did not sell enough copies. From my experience, most organizations have turned to Microsoft Active Directory for authentication services. Incidentally, *nix machines can authenticate against Active Directory fine, since it's just LDAP+Kerberos+some specific Microsoft extensions and enhancements. A few organizations use Orace Directory Services or FreeIPA. I use straight OpenLDAP at my workplace, and from discussions with various vendors, that seems to be fairly rare these days,
Note that the OpenLDAP documentation itself is fairly good, if you know how to look through it, and there are many how-tos on-line, but you're right there are very few recent books that I've been able to find, and I suspect that this is because there's not enough of a market for them.