I would like to nominate "Oliver Twist" by Charles Dickens.
Most peoples' knowledge of this probably comes from the 1960s musical, which I'm sure we've all seen innumerable times. The novel, though, is a VERY different beast, and definely written for an "adult" audience. It contains some wonderful writing, and has some unforgettable characters - Oliver himself, of course; Fagin, who runs a gang of child thieves and prostitutes, the professional criminal, Bill Sykes and his girlfriend Nancy, and many more.
This is a wonderful, unforgetable book, which everybody should read at least once in their life. I commend it to everybody.