Her best known work is probably her illustrations in The Chronicles of Narnia by C. S. Lewis.
She was also J. R. R. Tolkien's chosen illustrator: Farmer Giles of Ham, The Adventures of Tom Bombadil, Smith of Wootton Major, Tree and Leaf, and after Tolkien's death the poem Bilbo's Last Song (as a poster in 1974, as a book in 1990). She also painted the covers for the British 1973 one-volume and 1981 three-volume paperback editions of The Lord of the Rings, and produced illustrated versions of the maps from The Lord of the Rings and The Hobbit.