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By default REST query /_api/web/lists/getbytitle('<list title>')/items returns AuthrorId (User Id for Created By field):

{
    "d": {
        "results": [
            {
                //remaining output is omitted for clarity
                "FileSystemObjectType": 0,
                "Id": 1,
                "ContentTypeId": "0x010070092C478B02EF49ADA0691DE01FFBF6",
                "Title": "John Doe",
                "Email": "[email protected]",
                "ID": 1,
                "Modified": "2015-01-21T07:33:21Z",
                "Created": "2015-01-21T07:33:21Z",
                "AuthorId": 17,
                "EditorId": 17,
                "OData__UIVersionString": "1.0",
                "Attachments": false,
                "GUID": "01e58248-b0fc-4353-ad59-64010d9bb331"
            }
        ]
    }
}

Using $expand OData operator you can specify that the request returns projected fields from other lists and the values of lookups.

The following REST endpoint:

/_api/web/lists/getbytitle('<list title>')/items?$select=Author/Title,Author/EMail&$expand=Author

returns Title and Email for Author(Created By) column in list:

{
    "d": {
        "results": [
            {
                "__metadata": {
                    "id": "07849f26-815c-40df-ba3d-efaac658c066",
                    "uri": "https://contoso.sharepoint.com/_api/Web/Lists(guid'81916ced-b57f-4940-bf1a-eef4af00aaee')/Items(1)",
                    "etag": "\"1\"",
                    "type": "SP.Data.ContactsListItem"
                },
                "Author": {
                    "__metadata": {
                        "id": "b503837d-4cce-4b8f-96a4-26341abfc39a",
                        "type": "SP.Data.UserInfoItem"
                    },
                    "Title": "John Doe",
                    "EMail": "[email protected]"
                }
            }
        ]
    }
}