How, asks historian Anne-Marie Eddé, did Saladin, a “relentless jihad fighter,’’ come to be identified as a “valiant, generous, and magnanimous’’ figure among his former foes? Her weighty, comprehensive biography, “Saladin,’’ examines the birth and elaboration of a legend that casts a shadow even into the present day. In it, she highlights the conflict that can arise when our quest for historical truth runs up against the carefully constructed image that people of the past wanted us to see.