[{"_id":1,"books":[{"id":2,"pictures":[]},{"id":3,"pictures":[]}]}]

db.collection.update({"_id":1},[{"$set":{"books":{"$map":{"input":"$books","in":{"$cond":[{"$eq":["$$book.id",2]},{"$mergeObjects":["$$book",{"pictures":{"$concatArrays":["$$book.pictures",[{"description":"nice picture"}]]}}]},"$$book"]},"as":"book"}}}}])