Question: Robert Rankin wrote a novel based on what painting, housed in the Museo del Prado in Madrid since 1939?

Context paragraphs:
The Triumph of Death: The Triumph of Death is an oil panel painting by Pieter Bruegel the Elder painted c. 1562.  It has been in the Museo del Prado in Madrid since 1827.

Mona Lisa (Prado's version): The copy of the "Mona Lisa" is a painting of the same subject as Leonardo da Vinci's famous "Mona Lisa" (which is held by the Louvre Museum, Paris).  The copy painting has been displayed in the Museo del Prado in Madrid (Spain) since 1819, but was considered for decades a worthless copy.  However, after its restoration in 2012, the Prado's "Mona Lisa" was claimed to be the earliest replica of Leonardo's masterpiece.

Museo Nacional Centro de Arte Reina Sofía: The Museo Nacional Centro de Arte Reina Sofía (MNCARS, also called the Museo Reina Sofía, Queen Sofía Museum, El Reina Sofía, or simply The Sofia) is Spain's national museum of 20th-century art.  The museum was officially inaugurated on September 10, 1992, and is named for Queen Sofía.  It is located in Madrid, near the Atocha train and metro stations, at the southern end of the so-called Golden Triangle of Art (located along the Paseo del Prado and also comprising the Museo del Prado and the Museo Thyssen-Bornemisza).

Portrait of Camilla Gonzaga and Her Three Sons: Portrait of Camilla Gonzaga and Her Three Sons is a painting attributed to the Italian Mannerist artist Parmigianino and others, executed around 1539-1540 and housed in the Museo del Prado, Madrid, Spain.  It forms a pair with another painting in the Prado, the "Portrait of Pier Maria Rossi di San Secondo", Camilla's husband, a painting which is unanimously assigned to Parmigianino.

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.  It dates from between 1490 and 1510, when Bosch was between 40 and 60 years old.

The Grape Harvest: The Grape Harvest or Autumn (Spanish: "La vendimia o El Otoño" ) is a 1786 oil on canvas painting by Francisco Goya which depicts a man in autumn-colored clothes with his wife and son.  A peasant is presenting them with a sample of the year’s grape harvest.  The piece has been held by the Museo del Prado in Madrid since 1870.

The Garden of Unearthly Delights: The Garden Of Unearthly Delights is a novel by British author Robert Rankin.  Its title is a reference to the painting "The Garden of Earthly Delights" by the painter Hieronymus Bosch.

Equestrian Portrait of the Count-Duke of Olivares: Gaspar de Guzmán, Count-Duke of Olivares, on Horseback (Spanish: "Gaspar de Guzmán, conde-duque de Olivares, a caballo" ) is an oil on canvas painting by Spanish painter Diego Velázquez, made around the year 1636.  It has been in the Museo del Prado in Madrid since its inauguration in 1819.

Casón del Buen Retiro: The Casón del Buen Retiro is an annex of the Museo del Prado complex in Madrid.  Following major restoration work, which was completed in October 2007, it now houses the Museum's study centre (the Escuela del Prado) and library.

Museo de Arte Moderno (Madrid): The Museum of Modern Art ("Museo de Arte Moderno" or "M.A.M.") was the Spanish national museum dedicated to 19th- and 20th-century painting.  It was set up in 1894.  It closed in 1971, when its 19th-century collections were merged into those of the Museo del Prado, but housed at the Casón del Buen Retiro, and its 20th-century collections formed into the Spanish Museum of Contemporary Art (Museo Español de Arte Contemporáneo or MEAC), the predecessor of the present-day Museo Nacional Centro de Arte Reina Sofía.

Supporting paragraphs: The Garden of Unearthly Delights The Garden of Earthly Delights
Answer: The Garden of Earthly Delights