Matters

Information Retrieval (Z39.50): Application Service Definition and Protocol Specification is the latest copy (as far as I can tell) of the specification itself.

"Abstract: This standard defines a client/server based service and protocol for Information Retrieval. It specifies procedures and formats for a client to search a database provided by a server, retrieve database records, and perform related information retrieval functions. The protocol addresses communication between information retrieval applications at the client and server; it does not address interaction between the client and the end-user."