should read:

log_s = Stream.get("log");
log_s.record("append");

application.onAppStart = function(){
log_s.send("onLog", {event: "onAppStart",
time: new Date(),
appInstance: application.name});
}

application.onConnect = function(client, userName, password){
client.userName = userName; // Added this line to fix bug where userName was
undefined in onDisconnect.
log_s.send("onLog", {event: "onConnect",
time: new Date(),
ip: client.ip,
userName: userName});
return true;
}

application.onDisconnect = function(client){

log_s.send("onLog", {event: "onDisconnect",
time: new Date(),
ip: client.ip,
userName: client.userName}); // Changed userName to
client.userName to fix bug.
return true;
}

application.onAppStop = function(){
log_s.send("onLog", {event: "onAppStop",
time: new Date(),
appInstance: application.name});
}

Note that these errors are corrected in the downloadable examples.

In the source files the "sample9_5.zip" file contains the same files as the "sample9_4.zip" file.

Note from the Author or Editor:
Thanks for pointing this out! I've corrected the zip file on the book's site. Also, I noticed a related problem. On the same page this code is incorrect:

// Handle the result of calling areYouOk().
AreYouOKResultHandler.onResult = function(val) {
trace("Client " + this.clientID + " returned " + val);
}

The prototype object was omitted. The correct code is in the zip file and is:

// Handle the result of calling areYouOk().
AreYouOKResultHandler.prototype.onResult = function(val) {
trace("Client " + this.clientID + " returned " + val);
}