How to get User object using CSOM from SharePoint online list item?

newItem["Created_x0020_By_x0020_History"] = web.EnsureUser((((Microsoft.SharePoint.Client.FieldUserValue)(item["Author"])).Email).ToString());


                            newItem["Modified_x0020_By_x0020_History"] = web.EnsureUser((((Microsoft.SharePoint.Client.FieldUserValue)(item["Editor"])).Email).ToString());