Why was it a Book and not a DLC, it would've fit perfectly in the Anniversary of Oblivion, only way to get More Money out of People with the Same Broken Oblivion Game. :teehee:
Perhaps because many fans actually enjoy reading novels based on their favorite games? You will find novels based on many popular games including Dragon Age, Mass Effect, BioShock, Assassin's Creed, Deus Ex, Gears of War, Fable, Crysis and Halo. And as BootySweat said, it would have been a terrible DLC, The Infernal City is too large and ambitious for that. At the very least, it would need to be an expansion pack like Bloodmoon or Shivering Isles, and even then, it would have been a completely different experience. Games and books is 2 different storytelling mediums after all.
I don't think Bethesda wants the books to interfere with the games.
Then why release a book in the first place? I would expect the game to have, at the very least, a few references to the events in the novels. But i agree, the novel was not particular good, but it wasn't exactly bad either.