It's Lucas's editorial board that approves all the books. He himself sets specific guidelines (such as no writing about fall of Anakin Skywalker to Darth Vader, etc) and he has approved some specific things (I believe the loss of Chewbacca in the beginning of Vector Prime). Really its still owned by Lucas. Its his universe, he can change it however he wishes. He could have just allowed no one to expand it on it, then it'd be a pretty small universe. Sure there are various inconsistencies with all these sources, but it could be much worse. It's quite a job for the editorial board to keep things as consistant as possible - something I'd never want to manage myself.