There could be different design for the left and right pages of the book.

There could be a way to allow the content of a single page of the book to spread two pages.

Allow single page display.

Setting transparent background to RadBookItem doesn't work properly.

When a hard page is dragged and the mouse is released outside the Book range, there are flipping animation issues:

We start flipping the first (hard) page by clicking with your Mouse in the PageFoldArea. Holding the mousebutton we move the page over the middle of the book and outside of the books rectangle we stop holding the mouse button - the currently flipped page remains at the position it is, instead of completing the flipping process (although we released the mousebutton)…

Then we can move the mouse to the right upper corner of the next right page…
… move the mouse in the pagefoldarea and (before the hard page terminates its flipping process) click on page with index 2. Now the strange behavior starts. The book pages are now incorrectly displayed. If then you click on the right bottom page fold, parts of the coverpage suddenly reappear…
At a certain point the book control cannot be used than anymore.

Simulataneously working with multiple books needed.

Hard pages always can be turned over by single click no matter of the value of PageFlipMode

Possibility to have RadBookItems with different sizes (pages with different sizes)

When the Book is placed in a RadWindow sometimes its first page is flickering

When you switch too fast between a hard page and a soft page, you lose the ability to change the page through the fold or by clicking on it.

Currently, RadBook sets its RightPageIndex to 0 whenever a page is removed.

allow user to be able to set the pages to flip automaticaly

We have a RadBook control defined as:
<telerik:RadBook Margin="20" Height="400" x:Name="book" IsKeyboardNavigationEnabled="True">
<telerik:RadBookItem Content="Page1" />
<telerik:RadBookItem Content="Page2" />
<telerik:RadBookItem Content="Page3" />
</telerik:RadBook>

If we go to the last (third) page, the RightPageIndex becomes 2, and after that go back to the Page1 and Page2, the RightPageIndex is 0, which is incorrect.
The issue manifests itself ONLY if the keyboard is used for navigation. If pages are turned with the mouse - the RightPageIndex is correct.

Load on demand feature support similar TreeView.

When hover over the fold of the right(left) page and around the fold, sometimes the opposite -left (right) page is changing its content (the next page is shown)

Setting the RightPageIndex throws exceptions if the RadBook ItemContainerGenerator is not yet started

As a workaround you can set the property controlling the RightPageIndex in a dispatcher: 
this.Dispatcher.BeginInvoke(new Action(() => { dataContext.CurrentIndex = 2; }));

Another workaround would be setting the desired index after the containers have been generated. Use the ItemContainerGenerator.StatusChanged event and perform a check whether ItemContainerGenerator.Status is equal to the status "GontainersGenerated". If that condition is satisfied then setting the RightPageIndex should not throw exception.

Allow disabling smooth page dragging.

Allow pages with different size. Currently there are problems with the animation if the pages are with smaller size.

Allow page flip to begin not only from corners. See attached screenshot for further information.

Convert pages to WriableBitmap during page flip.