We have recently reviewed this bug report and it seems that with Silverlight 5, no flickering appears. Please have in mind that the Book control is not designed to work with transparent book items and generally such scenario is out of the scope of the control. However, if a similar issue is reproducible on your end, please contact us through the support ticketing system with details on your setup and a reference to this item. If a transparent background is needed for a specific use case, write to us with a follow-up referring the current item.

Regards,
Sia
Progress Telerik