Learn English – Word for “cinema with a single screen”

I think you could use the word surprising, which has one definition of "unexpected." So, your sentence would be: The progress was surprising.

I think that the word progress itself implies something positive. Therefore by saying that the progress was surprising, you imply that the result was both good and exceeded your expectations.

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”.)