I must have spent easily a week trying to figure out what I would put in here. Finally, I plumped for a two parter by Stephen King and Peter Straub. The Talisman and it's long awaited follow up, Black House are my choice(s) for adult novels.
As a child, my favourite book was the prequel to Redwall by Brian Jacques, Mossflower. Since I'm unable to read and enjoy them again as an adult, except when reading to them to my niece while we babysit, I've gone for one I can still read as an adult, as well. Coraline, by Neil Gaiman.