How to compute drag coefficient for falling book? The book measures 20 x 20.3 x 1.4cm and I'm assuming it's sealed shut so the pages don't flap open. Mass is 450g.

I want to understand how quickly it would fall if dropped straight down from 10km altitude. I've used a value for drag coefficient for a flat plate perpendicular to the air flow that I got from NASA (1.28) but I'm guessing that a falling book would spin round, slowing its fall, so this value is probably not right - is this likely?

So I'm wondering if it's possible to estimate the drag coefficient to any degree of accuracy. I don't have access to a wind tunnel and I'd rather not drop it off a tower as an experiment. :)
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.