Explanation:

- W.M. Davis was an influential American geographer, geologist, and meteorologist, most famous for his "cycle of erosion" theory. However, he is not the author of "The Unstable Earth."

- G. Taylor refers to Griffith Taylor, an Australian geographer known for his work in environmental geography. He did not write "The Unstable Earth."

- J.I. Clark was an earth scientist who contributed to geological sciences but is not associated with the book in question.

- J.A. Steers is the correct answer. He was a prominent figure in coastal geography and authored "The Unstable Earth."