LDAP, the Lightweight Directory Access Protocol, is the open standard successor to NIS and other methods of distributing directory information. Useful applications for it are passwd/group/aliases replacement, including virtual mail accounts, phonebook and printer configuration. That said, you're as likely to have its implementation imposed on you, by an application that `must have it', as to choose to do it yourself.
OpenLDAP is the standard LDAP server implementation in Penguin-space; Sun ONE directory server (was Netscape Directory Server) is the standard commercial implementation, while Novell's Netware Directory Services is `LDAP Compliant' and Microsoft Active Directory Services (ADS), `embraces and extends' LDAP.
This book focuses on implementing LDAP using OpenLDAP v2, the Open Source implementation (its license is BSD-ish) and connecting it with client services.
The first two chapters deal with the basics of LDAP, the schema and the schema definition language. The next three lead you through installing OpenLDAP, OpenSSL, a DBM database, and SASL, and creating a toy applicaton, a "company white pages."
The last half of the book deals with making it useful: interoperating with real applications. Chapter 6 interfaces LDAP with PAM, NSS, and NIS to provide authentication services. Chapter 7 addresses email, including clients (Mozilla Mail, Eudora, Pine, Outlook) and mail transfer agents (Sendmail, Postfix, Exim). Chapter 8 goes on to embrace other UNIX services: FTP, httpd, Samba, RADIUS, DNS, LPRNG/LPD. Chapter 9 gets into the issue of interoperability, including Windows Active Directory Services and Kerberos, and Chapter 10 is devoted to using PERL to manipulate LDAP.
This book is a good practical guide to implementing LDAP which doesn't minimise the amount of work you're facing.