Well, putting it where the route changes would probably violate single responsibility.  There's really no reason for the view to have to be calling some function to make it fetch in the view either (and if you did put it in render, you'd need to include some logic to make sure it only happened once).

When you navigate to a new book, it's going to create a new view that will also instantiate a new `content` model, and since you know you want the model to fetch only once and to do it essentially as soon as it's created, `initialize` would seem to make the most sense.