How to compute drag coefficient for falling book?
You have a very thin book. The answer depends on how you initially drop the book.
1. If you drop it flat about its front
It will very closely approximate a flat plate; in this case, the pressure drag will overwhelm the skin friction drag. Hoerner, Fluid Dynamic Drag has some empirical data on the drag coefficient of a bluff body (near 2.0):

To retrieve the 3D coefficient, dimensionalize it with the frontal area.
2. If you drop it broadside
The skin friction drag would dominate. You can approximate it via the flat plate friction drag. For a single side, the drag coefficient looks like (from Anderson, Fundamentals of Aerodynamics):

3. If it tumbles
Your book may also tumble at some point due to random perturbation about its major principal axis. This changes the drag even further; but once again, Horner has some empirical correction formula.