Each book does have to earn its right to exist.
But you’re picking at ghosts here.
I might argue that the reason you can’t find NOVA volume 3 is that it sold well–well enough to sell through its print run more quickly than anybody could have anticipated.
Just take a look at all of the Carol Corps activity on tumblr, and the get-togethers at conventions, and the cosplay events. And then ask yourself: where do you think most of these particular readers are getting their copies of the book? (If your answer was digitally, you’d win the gold star!)