What makes a book unforgettable for you?

[~Jennifer~]

I've read a few books in the last year or so that have lingered in my mind to the point of insanity. I like to think that I'm not completely nuts so I'm hoping I'm not the only one. Does this happen to you? What qualities in a character/book make them stick in your head?

[Shellbelle]

I used to be a lot pickier in my books because I wanted every book I read to leave me with that same feeling...the feeling you get when you actually a little depressed because your done with the characters and the world, and it's hard to get them out of your head! Now I read a lot more variety but I still LOVE when I come across a book or series that does that for me.

Right now I'm reading and ALL of Cassandra Clares characters and worlds do that to me! But I honestly can't pinpoint what it is that does it for me...