What if some libraries could be pushed to open secret passages? Of course, not each library would act like a secret door, there would be two different types, maybe the secret door would have a darker color, or the player could just guess there is one because the map is incomplete.
The move-able library can be pushed to make a path toward the secret room, or maybe destroyed or open like doors.
Maybe the Wizard and Ranger can not push it normally and need to use a book or wand to push it (cant remember the wand´s name), since the Warrior have more strength he would be able to move it freely.
Maybe some breakable walls can be added as well, as secret rooms.