Undone — Cat Clarke

Jem Halliday is in love with her best friend. It doesn't matter that Kai is gay, or that he'll never look at her the way she looks at him. Jem is okay with that. But when Kai is outed online by one of their classmates, he does the unthinkable and commits suicide.

Jem is left to pick up the pieces of her broken life. Before he died, Kai left her twelve letters -- one for each month of the year -- and those letters are all Jem has left. That, and revenge.

Although Kai's letters beg her not to investigate what happened, Jem can't let it go. She needs to know who did this, and she'll stop at nothing to find the person responsible for Kai's death. One way or another, someone is going down. Someone is going to pay.