Please always include the skin. I for one can't remember who is using what. 🙂
I can't see settings in the obvious skins that allow you to change what shows from anything other than top level books. If there were such a setting it would apply to all lower level books so are there other examples?
Have you tried deleting it, generating and then adding it back?
Do any of your books have topics linked? It shouldn't cause this behaviour but worth testing.