If you need a book with a custom pages, for example with rounded corners or with thickness and so on, just follow these steps:

1.

Go to the Layer Manager and make the hidden layer visible.

2.

Create your own geometry for the page (with rounded corner, with thickness, …)  and make it child of the Piano geometry. Adjust the position and the dimension (depend on your poligon).
3. Now disable the Piano geometry.
4. Return on the Layer Manager and hide the layer again.
5. Duplicate the page.
6. Finish.