Now, BATTLEFIELD EARTH, although a fairly long book, was, I thought, a pretty good read in a post-Armageddon Alien Invasion Campbellesque-SciFi Swashbucker kind of way: outnumbered, innovative Earthlings battle against all kinds of alien invaders and, because of their pluck and daring, succeed in overthrowing the inter-galactic status quo.
I only made it one and a half book into “Mission Earth”, and yes, unreadable.
I thought “Battlefield Earth” was somewhere within those 12-13 books.