Kirby began planning his "Introduction to Entomology", a celebrated title, in 1808. This was the practical result of a friendship formed in 1805 with William Spence, of Hull, and appeared in four volumes between 1815 and 1826. Much of the work fell to Kirby owing to Spence's ill health. The book was illustrated by John Curtis. It reached its seventh edition in 1856. In 1830 he was invited to write one of the "Bridgewater Treatises", his subject being "The History, Habits, and Instincts of Animals" (2 vols., 1835).