Are books really necessary under my head? Why not a pillow?
Books provide support for the neck if there is chronic downward pull (neck tension).
The goal is to encourage a gentle traction on the muscles of the neck. Some days when experiencing lots of stress, you may need a high stack of books. While on other days, you may already be very free and need only a small one or even none.
Books offer more actual support than a pillow, and in finer increments. Pillows are also a stimulus for a different kind of rest, called sleep.