The Amber Spyglass by Philip Pullman

“You have to promise to believe me,” Lyra said seriously. “I know I haven’t always told the truth, and I could only survive in some places by telling lies and making up stories. So I know that’s what I’ve been like, and I know you know it, but my true story’s too important for me to tell if you’re only going to believe half of it. So I promise to tell the truth, if you promise to believe.” -Lyra "Silvertongue" Belacqua