The first entry in Del Rey's upcoming "Rebels" book series will be Martha Wells' novel starring Leia Organa. Here's how the Edelweiss catalog describes the book:
Nebula Award finalist Martha Wells makes her Star Wars debut with a brand-new classic Star Wars: Rebels adventure starring Princess Leia and her new friends, Luke Skywalker and Han Solo, in the time just after the destruction of the Death Star in Star Wars: Episode VI A New Hope!
Princess Leia Organa is on a mission for the Rebel Alliance when Imperial forces attack. Now she, Han, and Luke are on their own, working with pirates and dealing with traitors as they race the clock to protect the secrecy of a crucial meeting of Rebel conspirators!
The book is currently listed with an October 29, 2013 release date.
Also in the Leia novel's catalog entry is an overview of the release time frame for the "Rebels" series. James S. A. Corey's Han Solo novel will be released in March 2014 and Kevin Hearne's Luke Skywalker novel will be released in January 2015. Of course, these books are still a long way out, so release windows could (and probably will) shift somewhat as we move forward.