If you apply a filter such as Sepia or B&W from the viewer's Photo Filter menu to a book image, it will appear without the filter when viewed under the Loupe.
This happens because the Loupe looks at the source version of the image, and book filters only affect the book itself—not the source. If you apply a similar filter to a version of your image using the Adjustments inspector, you'll see the effect when you view the image under the Loupe.
Here's where to find the Photo Filter menu in a book Viewer:
