What are some of your favourite Superhero (NOT Graphic) Novels? There is something about the nature of well written prose that can help you get inside a character's head in a way that surpasses the comics medium.
I still think of Elliot S! Maggin's Superman Last Son of Krypton and Superman Miracle Monday as the most iconic depictions of Luthor and Superman for me.
He also did a great job on the Kindom Come Novelization.
I loved Fantastic Four: Doomsday. For me, this is the ultimate Doom storyline.
What was neat about Peter David's prose Hulk stories was the way it "fit" into the continuity he built in the Hulk comic and specials projects such as Hulk: Future Imperfect and Hulk: The End.