A fictional book is a non-existent book created specifically for (i.e. within) a work of fiction. This is not a list of works of fiction (i.e. actual novels, mysteries, etc.) but rather imaginary books that do not exist.
So, there is such a thing as a non-fiction fictional book. Interesting.
And how about a non-fictional book? Is that just a real book?
My favorite fictional author: Julian Carax.