I loved "One Day" by David Nicholls and was very excited to see him speak last year. I also fell a bit in love with him. He was kind of cute. It's a corker of a story and spoke to me about lots of stuff I could relate all too well to, sadly, like addiction, messing things up and unrequited love.
I tried Lionel Shriver's subsequent and previous novels and they're not a patch on this one. This book shocked me so much that I had to read it again a week later. I'd definitely suggest reading "We Need to Talk About Kevin" before seeing the film which is due to be released soon.
Donna Tartt's masterpiece, "The Secret History" is deservedly a cult classic. This book is so dark it should come with a night light.
Obvious choice, but "The Catcher in the Rye" really is a moving book and a pure joy to read. It's considered a classic for good reason.