I found that the Comic Book Reader suffers the same CTRL-TAB problem that I previously experienced in Edi.
The solution to getting CTRL-TAB to work as expected is to set a default keyboard focus within the document view. One simple solution is to have a textbox and the FocusManager acquire focus on that textbox by default.
Documents usually have no textbox so I found a better solution, which is an attached property (see attachment). You can use it like this in your view:
... UserControl x:Class="Edi.View.StartPage" ...
Your app is pretty cool by the way so I hope this improves the experience for those who use the keyboard (as I often do...)