This is a compilation of YA novels told completely in text messages and IM

PS Longer Letter Later and Snail Mail No More by Ann Martin and Paula Danziger.
Bad Kitty ChaseR by Michael Rosen
Naked Mole-Rat Letters by Mary Amato
Rob&Sara.com by P.J. Peterson and Ivy Ruckman
Donorboy by Brendan Halpin.
Skeleton Creek by Patrick Carman.
Entr@pment: A High School Comedy in Chat by M. Spooner.
>From E to You by Chris D'Lacy and Linda Newbery
Heart on My Sleeve, by Ellen Wittlinger.

There is a list of fiction books that incorporate digital communication for teens in the September 2008 Book Links, p. 57-60, that may be useful. Was put together by Melanie Koss.

Thanks to Diane Herald for this list.