Diamond Mask by Julian May
Earth is finally part of the Galactic Milieu. But the next step is to achieve Unity with other galactic races. To attain this higher state, we must gather sufficient numbers of the metapsychically talented. However, a rebel group want to keep Earth forever separate. And their covert supporter is Fury, an unstable metapsychic entity with its own ruthless plans for humanity. Fury's greatest threat now is Jack the Bodiless, whose power it craves. But Jack won't be used.
So the entity turns to Dorothea Macdonald, a young woman who's been hiding her great powers from the Milieu's best mind-readers. Yet she can't keep them from Fury - or Jack. She rejects their advances, suspicious of megalomaniacal dreams and galactic politics. But when Jack's brother Marc puts his schemes ahead of saving millions of lives, Jack needs her. Because ultimately no one - even the Galactic Milieu - can match the powers of the girl they'll call Diamond Mask.
'Rich in intrigue and vibrating with creative energy' Library Journal, 'May has largely earned her impressive following, which makes this book a virtually mandatory acquisition' Booklist