Same question!

I have sent an email to that address. I also sent a ticket to Developer Support, as adviced by Apple Support on Twitter. I have not received a response from either yet.

I asked Esther Hare, who is Apple's Worldwide Developer Marketing Director, in Twitter regarding the usage of PlaygroundBooks. She replied and said the same thing as what the Staff member here says. Bummer.

So to clarify, we aren't allowed to submit Playground Books?