By random, I mean one that you pick up in a charity shop or one that someone gives you, by an author you're not familiar with.
I recently picked up The Namesake by Jhumpa Lahiri. I'd never read anything by her, and because the book cost 10p at a scout jumble sale, I wasn't expecting it to be such a great read. I've now ordered another couple of books by her.
I love it when a book exceeds your expectations.