David Day was born and brought up in Canada. He has published many books in the fields of poetry, ecology, natural history, fantasy and mythology and a number of award-winning children's books. His writing has also appeared in many magazines and anthologies. His enduring fascination with complex mythologies led him to J.R.R. Tolkien and Day is best known for his best-selling books on the life and works of that author. It all began with the publication of A Tolkien Bestiary (1979), a lavishly illustrated book that for the first time gave readers of The Hobbit and The Lord of the Rings some indication of the immense scope of Tolkien's mythology and cosmology.

Day's subsequent books include Tolkien: The Illustrated Encyclopedia (1992), The World of Tolkien: The Mythological Sources of Lord of the Rings (2002). An Atlas of Tolkien and its companion volume, A Dictionary of Tolkien, were the first two books of a seven-volume reference library and more recently, The Illustrated World of Tolkien (2019) and The Illustrated World of Tolkien. The Second Age (2023), celebrate more than 40 years of Day's works and delve into the lesser-known parts of Tolkien's legendarium.