1969’s The Face in the Frost is the first novel in John Bellairs’ Prospero and Roger Bacon series. The edition I own was illustrated by Marilyn Fitschen.
The wizard Prospero — not the Prospero of whom you are thinking — makes his home in an insignificant island nation. His cozy home is filled with sorcerous paraphernalia, including the obligatory magic mirror. He is visited from time to time by his English friend Roger Bacon — almost certainly the one of whom you are thinking. Prospero enjoys a comfortable life free from petty worries.
Enter the existential threat.