Everything, Everything by Nicola Yoon

This is the story of the thrill and heartbreak that ensues when we break out of our shell to do crazy, sometimes death-defying things for love. Madeline Whittier is allergic to the outside world. So allergic, in fact, that she has never left the house in all of her seventeen years.

But when Olly moves in next door, and wants to talk to Maddy, tiny holes start to appear in the protective bubble her mother has built around her. Olly writes his IM address on a piece of paper, shows it at her window, and suddenly, a door opens. Maddy is certainly going to fall in love with Olly.

It's almost certainly going to be a disaster.