"Pisanie ksiazek" and "writing a book" are different things.
"Pisanie książek" is "writing books", not "writing a book".
This seems to be a ghost sentence that was deleted from the course, but they tend to show up from time to time anyway.
And it's still here.
We have only "Writing books is a difficult process." in the tree... with singular versions accepted, but not starred.
What's wrong with "Pisanie książki jest trudnym procesem."
Nic. Powinno być uznane za poprawne.