“Much is being said this year about “Don Quixote,” in celebration of the 400th anniversary of its publication. And indeed, much has always been said about this extraordinary epic, narrating the misadventures of a half-mad hidalgo who seeks to re-establish the traditions of knight errantry. Faulkner reread it annually; Lionel Trilling said all prose fiction was a variation on its themes. But aside from its literary achievements, “Don Quixote” sheds oblique light on an era when Spain’s Islamic culture forcibly came to an end.”