So thrilled to hear you enjoyed WHEN, (this is the new title for DEATH DATE, so I'mma use it to reply to you, if that's okay?) :) I'm also so pleased that you were shocked by the ending. I worked very hard on that twist and I'm always delighted when people tell me they were surprised, so thank you. :)
To answer your question, yes, I will be writing more YA, (yay!) I have another book under contract with Hyperion, and that is currently set for release in early 2016. That book will explore a topic I'm personally so curious about - reincarnation, and the tentative working title for that novel is FOREVER...AGAIN. Hope to give you another surprise ending with that story, too. (Twisty endings are the bestest, no?) :)
Thanks again for your question and for your lovely praise.