A novel/book equivalent of the term off-screen

Before we had the (silver) screen, we had a stage.

Something that was not visible to the audience happened offstage, which freedictionary describes as:

adj. 1. Situated or taking place in the area of a stage that is invisible to the audience.
adv. 2.b. Behind the scenes; not visible to the public: The meetings between the leaders took place offstage.

I see no problem applying this to a book, the stage idiom being quite commonly understood and quite adaptable (offstage can also refer to non-theater settings, meaning in private, as opposed to the part of one's life that takes place “in the spotlights”.)

That's a good question, and one for which I've never heard the answer (at least that I can recall). You could perhaps describe this as events occurring "outside the narrative or discourse."