At the end of the ledge, left from the doorway where you entered after finding the treasure, is a stairway leading down. Follow it to a gap in the floor. Use Indy's whip to swing across, then climb the ladder down to the next level. Turn left, jump up and pull yourself up to the ledge above. Be ready to shoot a guard soon as you get to the top.
Go straight ahead, past an intersection to the left, to the end of the corridor. Turn right into a dark room and get treasure 2 silver coins. Go back to the intersection you just passed and turn right. Angle left and climt the ledge. Go trough the doorway by the crate with the desk lamp and book on it.
I hope it helps you, succes