Backbone.js - where to store state information?

Don't limit your Backbone apps to Backbone constructs. If you find that your app needs some functionality that doesn't fit well into one of Backbone's constructs, don't shoehorn it into one just for the sake of keeping everything in Backbone.

I've found this backbone boilerplate to be helpful in this respect. It sets up modules for you and provides you with an app object that extends Backbone.Events (like in the previously-linked article). This app object can be used to store instantiated models, views and controllers/routers, and you should feel free to add your own non-backbone modules to the app object, to take care of responsibilities that don't fit perfectly into one of the Backbone constructs.

Why don't create a state model to store and describe the current state? I don't think that is wrong. Since the current state involves more than one model I think it's reasonable to create a state model to store and receive the current state.

The controller could then communicate with the state model to get the current state. The UI states should be stored in the corresponding model though. The state model knows which book and which page and then the book and page models keep track of their current UI states.