Note: Each book in the series is published in 4 different editions with (almost always) consecutive ISBNs in the set:

1. US paperback
2. UK paperback (including different title for US/UK difference). See The Siren and the Specter/The Siren and the Spectre for an example.
3. US hardcover
4. US e-book

It is possible that there are additional UK titles (hc+e-book?) and an audio book. Further research is being done on the series and publisher.