Cover of the Burgos edition, 1499.
Properly speaking, this is not an actual cover, but rather an illustration of the first scene of Act I, depicting the initial encounter between Calisto and Melibea. The cover, if there was one, has been lost. For this reason, the first illustration is now often considered as the cover, even though it is not a proper cover in the strictest sense. This is an edition with 17 illustrations which can be seen by clicking here: Burgos, 1499.
Rojas, Fernando de. Comedia de Calisto y Melibea. Burgos: Fadrique de Basilea, 1499