Where has the artwork referenced by the title of the novel "The Garden of Unearthly Delights" housed? 
The Garden of Unearthly Delights: The Garden Of Unearthly Delights is a novel by British author Robert Rankin. The Garden of Unearthly Delights:  Its title is a reference to the painting "The Garden of Earthly Delights" by the painter Hieronymus Bosch. The Garden of Earthly Delights: The Garden of Earthly Delights is the modern title given to a triptych painted by the Early Netherlandish master Hieronymus Bosch, housed in the Museo del Prado in Madrid since 1939.
Museo del Prado in Madrid