The GXP novels are essentially the producer's cut of GXP.
Due to the nature of anime series being only 26 episodes, Kajishima felt some material had to be cut, and then his director didn't help the situation he glossed over a lot and overemphasized the humor ignoring the plot. So when it was done he felt the need to do it again properly and now it's the version of the story the story uses.
When you read the novels you will notice that things which show up in the anime very slightly are now fully developed and explained some were things you never even noticed before, but they're in the books.
The entire series has been translated in abridged form by me and currently a second revised translation is being done and is about half complete
The OVA 4 Novel should be read between novels 4 and 5. Ova 5 and Paradise starting novels are after the Renza ark but before war on Geminar.
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