If the book is not only opaque but also non-white then it means that some of the wavelenghts of the visible spectrum are being absorbed by the book. That absorbed light converts into heat, so we can use the book to boil some water and make a steam machine work. Since the book does not cast a shadow then it means that a similar book could be placed behind him and still get all the ligth on it. So you could make layers of books, all absorbing that energy several times in a stack. Wouldn't ir be wonderfull to have some sort of solar panels that can be stacked one in fornt of the other? You could increase the amount of energy gathered per square meter by any factor you want.
It would also be awesome for CCD technology: each photon could be detected on each of the books (maybe sensing the minute increase in heat on the book) and it could still travel to the next book, This would make easy to statistically confirm each signal and noise on any picture would no longer be an issue.
This is just an insignificant portion of what could be done. When you break the laws of physics it turns out you can almost do anything. Violating the conservation of energy like that would have several theoretical implications. For example, thermodynamics would be completely different, perpetual motion machines could be done, etc... Another interesting thing is that Noether's theorem would then imply that we have no translational symetry in time for any mechanical system, which would mean that the laws of physics would appear to change as time goes on.