The Devil's Day(1985)
(A book in the After Such Knowledge series)
An omnibus of novels by James Blish
A cloistered monk desperately tries to close the Pandora's box opened by a mischievous weapons dealer who has recruited a powerful black magician to stir up trouble for humanity on the eve of Judgement Day.
Used availability for James Blish's The Devil's Day
May 1985 : USA Mass Market Paperback