Howe did this in 1989, so actually he was the one directly riffing on earlier artists' versions of Gandalf, like Hildebrandt, Bakshi and Rankin-Bass. Or just iconic images of wizards that go back centuries...
I had a copy of Lord of the Rings with this as the cover art. Unfortunately, the bookshop decided to use those unreasonably sticky price labels that never completely come off.