Why not use the new beta reader with Books?

I was kinda confused why the Books use the old reader when I have the beta enabled for subscriptions. I can't see why they don't use the same reader. :)

Thank you.

Maybe because while reading the books you can jump chapters and at the moment the beta reader wouldn't support that function

We'll be making the subscription reader available to books soon, just working our the final bugs.

Making the new reader work with books requires some additional work which simply isn't complete yet. But it's definitely coming.