: The entire 9-book series is available on Amazon , with many titles included in the Kindle Unlimited subscription for free reading.
: Most books in the series have audiobook versions narrated by actors like Annie Ellicott and Jeff Hays. : The entire 9-book series is available on
: This collection includes the first four volumes of the series: The Laboratory , The Airship , The District , and The Metropolis . and The Metropolis .