1. Parasite Eve
Parasite Eve is both a personal favorite and the most interesting case I could find. There is, indeed, a novel called Parasite Eve written by Hideaki Sena, but the video game isn't an adaptation of the novel: it's a sequel! Man, the video game industry was so different in the '90s, huh? Little did I know all those references to a prior "event" in the game were actually references to a previous work rather than just a random plot device. If you haven't heard of it, Parasite Eve is a horror-action RPG published by Square-Enix. The story concerns NYPD rookie Aya Brea who may be the only person who can save New York from a mysterious being that calls herself Eve. Eve is able to control mitochondria, the cell's "power station" as it's sometimes called, which gives her the ability to burn and liquefy anyone she chooses. The overactive mitochondria causes rats, dogs, and other creatures to undergo rapid and frightening mutations. While anyone can put bullets into the monstrosities, only Aya can approach Eve without turning into flaming Jello. I'll let you discover why on your own. The game plays as a fairly straightforward JRPG, but with a few differences: the game's story takes place over a period of a six days across various locations in NYC. You're free to go to any location on the city map, though you have to visit certain places to advance the plot. Parasite Eve's battle system is based on random encounters, but there is no separate battle screen like the Final Fantasy games, for instance. The game does, however, borrow Final Fantasy's ATB (active time battle) bar. You're free to move around during battle to position yourself for attack or dodge enemy assaults, but you can only take an action when the bar is full. Aya can attack with her currently equipped weapon, use her "Parasite Energy" (essentially this game's version of white/black/time magic), change equipment, or use items. Finally some good news for people who like reading: Parasite Eve is easy to get in the States. It's available on, you guessed it, Amazon.com for just under 12 bucks. The games are also available on the PlayStation Network.
Did any of these books-to-video games surprise you? Know of another one I didn't mention? Let us know in the comments section.