It's a multivariate system, the dragforce is determined by the car geometry + book, hand, whatever.

They are both causes to speed.

In fact you don't even need flow to infer speed. You can just use pressure calculations and temperature, which is how airplanes measure their speed.

Controlling drag is a major component of the inputs to speed when flying an aircraft.

I can't tell if this is germane to the analogy or not, but the air flow over your hand, even behind a book, is still a function of the car's speed. As an illustrative example, imagine the feeling when the car is at 0 compared to when the car is at 60.