Book name - - since the technique is for any object, not just portraits, I would suggest using just the name "Silhouettes". One of the definitions of Silhouettes is: Shadowed contour – an outline of somebody of something filled in with black or a dark color on a light background, especially when done as a likeness or work of art.
With all the suggestions you are getting, it is probably more difficult for you to come up with a title and book cover than the technique that the book will be all about.