There is rhythm in movement of the sea and tides, stars and planets, trees and grasses, clouds and thistledown. It is part of all animal and plant life. It is the movement of uttered words, expressed in their accented and unaccented syllables, and in the grouping and pauses of speech. Both poetry and music are the embodiment, in appropriate rhythmical sound, of beautiful thought, imagination or emotion. Without rhythm there could be no poetry or music. In drawing and painting there is rhythm in outline, color, light and shade.
Bridgman’s Complete Guide to Drawing from Life
Drawing books don’t get much better than this.