✔ 最佳答案
Carbon Monoxide,
Basically the gas that's used in a gas oven :)
If you just leave the gas on till it fills the room, you eventually just fall asleep, and then suffocate painlessly in your sleep, look it up!
Most ovens these days cut the gas off if there's no flame though, so if you wanna put it in a book, make sure it's an old oven or something :)