Books that you carry around with you for weeks just rereading them, books you tell your mates about (and then if you see someone reading the book you rush up and start a conversation with them because you know they won't mind) - books that, when you meet a person and find out that this person has read and liked that book you know you will be friends.
Ok here are some of mine
Robert Heinlein: Time Enough For Love
Easton/Lizt: The Ethical Slut
Paul Breeze While My Guitar Gently Weeps
ANy Christopher Brookmyre (OK that is a bit of a cop out as so many people have read him)
Gwyneth Jones Bold as Love series.