Yes, this book is published in a horizontal format. And that has a reason why.
This story was originally published in panelsyndicate.com, a web page that you may not know about originally created by Marcos Martin and Brian K. Vaughan to publish this book we’re talking about. After that, it has also published creator owned books from other authors to be read online, in a pay-as-you-wish format.
This means that you can download the books by paying as much as you can or want, to read in your computer. In your screen. That, by the way, looks horizontal. So, that’s the reason of the format.
All in all, an interesting idea… the authors behind the books they publish are great, so it’s a really interesting site to check.