A description of the importance of illustrations in scientific books appears elsewhere on this blog. Illustrations help convey a writer’s thinking. However, I must mention a notable exception.
The work of the Nobel prize-winning ethologist Konrad Lorenz (1903-1989) is notable for not being illustrated. (According to a colleague at the Konrad Lorenz Institute.) Indeed, flicking through some of his books, except for one table of figures, all I saw was prose.