The first novel to be released is the Star Wars Rebels novel "A New Dawn." More information about the new canonical books can be found at StarWars.com.
"The first novel to benefit from this deeper collaboration is Star Wars: A New Dawn, by bestselling author John Jackson Miller. Set prior to the events of the forthcoming animated series Star Wars Rebels, this novel tells the story of how two of the lead characters of the series, Kanan Jarrus and Hera Syndulla, came to cross paths. To tell this important backstory, Miller benefited from contact with series executive producers Dave Filoni, Simon Kinberg and Greg Weisman, who together ensured this tale will be part of the Star Wars canon of storytelling going forward. It is scheduled for hardcover and eBook release on September 2, 2014."