Add request without progress

[flyer2001]

In some cases (for simple request) no progress is needed

[Alexander-Ignition]

You can write an underscore if you don't need progress

_ = client.request(endpoint) { (result: Result<Book, Error>)
    print(result)
}

or do you want the function request(_: completionHandler:) to be with the annotation @discardableResult?