In Breaking Dawn, Bella and Edward finally make a deal: Edward will turn Bella to a vampire and she'll marry him first. Even though at first, Bella doesn't want to marry, she soons finds that she'll enjoy being Bella Cullen. Before turning, she goes on her honeymoon with Edward. It's a huge shock when she ends up pregnant.
Jacob, depressed and roaming through the forests, when he finally decides to return home. He can't resist wondering about Bella, so he finds her at the Cullens, and is mortified when he sees her dying from pregnancy. The child is too powerful, considering that it's probably a vampire, and she's getting weaker day by day. She won't allow the child to be removed, because she already loves it. Edward is also depressed because he let it happen.
This book closes up all the lose ends from Twilight, New Moon, and Eclipse. Written in three parts, having the second in Jacob's perspective (unlike the other books), Breaking Dawn also provides further information on vampires, werewolves, and the Volturi. Plus, you get to see a side of Bella that's never been seen before, the opposite of being protected.