How to Read Values From Related Tables

// Basic retrieve
Xrm.WebApi.retrieveRecord("contact", customerId, "?$select=firstname").then(
    function success(result) {
        console.log("Retrieved values: Name: " + result.firstname);
    },
    function (error) {
    	console.log(error.message);
    } 
);

// Using expand
Xrm.WebApi.retrieveRecord("contact", customerId, "?$select=firstname&$expand=modifiedby($select=fullname;$expand=businessunitid($select=name))").then(
    function success(result) {
        console.log("Name: " + result.modifiedby.fullname);
    },
    function (error) {
    	console.log(error.message);
    } 
);

Source: retrieveRecord (Client API reference)