Uglies. Scott Westerfeld.
This series blew me away.
It is a trilogy-plus-one, meaning that the first three books all involve the same characters that are continuous, but there is another book afterward in the same world and the same characters are probably mentioned (I have not read the fourth one yet) but it deals with different things or perhaps characters.
The first book, Uglies, was recommended to me by two different people. At first, I was kind of wary. I had no idea what to expect, and it took me a while to understand the gist of the story, and how it is futuristic and they have many, many cool things that we do not have. All very future-like things, obviously, since it takes place hundreds of years from now. Now, I have never read a formal futuristic book. But I LOVED this! I was warned that it may take a while to get into, but that did not happen whatsoever. After I basically understood it, it was breathtaking. And the chapter cliffhangers were always very good, but it wasn't until near the end that a HUGE cliffhanger that made me gasp came. I won't spoil anything for anyone who hasn't read these yet, but I HIGHLY recommend them.
Basically, there are different stages of life. Littlie, Ugly, Pretty, so on. Once the people of the city turn sixteen, they get an operation that turns them pretty. Tally, the main character, wants nothing more than to be pretty. She spends all her ugly time dreaming of being pretty. She meets Shay, the crazy best friend. Shay shies away from the idea of being pretty. Then, just before their shared sixteenth birthday, Shay tells Tally about running away and where she can run to so they won't get the operation. After a turn of events, Tally has two options: She can turn in Shay to the authorities and turn pretty, or she can stay ugly forever.