There is technically one last major book for Middle-Earth that Peter Jackson could technically tackle if the Tolkien estate suddenly gets over their hate for him and the movies (which will never happen). It is called The Silmarillion that acts as a kind of encyclopedia (with all the facts both interesting and boring that word implies) of the Middle-Earth universe. It gets into the nitty gritty details of Middle-Earth including the origin story of the races and some of the characters we now know from the films that the below four minute video does an excellent job of explaining (hopefully it is a first of many that can provide the Middle-Earth details without slogging through The Silmarillion).

The Origin of Middle-Earth Explained. There are any The Origin of Middle-Earth Explained in here.