Depends on the story, I guess. The DGM novels give background stories to the main story. The funny thing is that the main stories of the two first novels got animated as "fillers", but they aren't "fillers" per deffinition of "filler", as they're actually canon. |D
I don't mind reading either, be it a book or- as I recently discovered- Visual Novels.
Then a lot of the DGM "fillers" aren't "fillers" according to your definition. I like that definition. : > I love background stories. Episodes for the sake of fanservice, for example, are definitally filler.