Dustfinger looked first at Mo, then at Meggie with an expression of incredulity on his face. "You haven't told her?" Meggie heard him ask in a low voice. (3.27)
Earth to Meggie: your entire childhood has been a lie, and now this stranger turns up on your door and he knows the truth while you don't. Have fun with that revelation.
Mo nodded. "One of my library commissions has been postponed—you know how libraries are always short of money."
Meggie looked at him uneasily. She hadn't realized he could lie quite so convincingly. (4.29-30)
This is a fun realization: your dad is actually quite a good liar, and he doesn't hesitate to lie even to other family members. Of course by now Meggie knows that her dad has been keeping secrets from her (about a mysterious man named Capricorn, and about a book that she's not allowed to see), so maybe this doesn't come as such a huge surprise after all.
When she was little she believed that Mo had simply invented a mother for her one day because he thought she'd have liked to have one. He told wonderful stories about her. (9.2)
Mo has told some really fabulous and inventive stories about Meggie's mom, the adventures she's off having, and excuses for why she can't come home and be with Meggie. It's almost become normal for Meggie to have a made-up image of a mother. Then again, most of us form images of the people in our lives that don't quite correspond to reality.
His scarred face only reminded her of the lies he had told to lure her out to him, like the Pied Piper in the story. He had played with her as he played with fire, with his brightly colored juggler's balls: Come along, Meggie; this way, Meggie; trust me, Meggie. (17.28)
Meggie realizes that Dustfinger played her, and she feels stupid and gullible for believing his lies. No one likes realizing that they've been manipulated, and Meggie is no exception. It makes us wonder: Is Dustfinger a better liar because he's already good at manipulating physical objects like juggling balls and fire?
"You're such a gullible fellow, Dustfinger. It's fun to tell you lies. Your innocence always amazed me—after all, you lie very cleverly yourself." (17.58)
Capricorn is just toying with Dustfinger here, gloating about how easy it was to lie to him. Lying to Dustfinger is part of what helped Capricorn get his hands on the book, Mo, and Meggie, but it seems like Capricorn is also into lying for the sheer pleasure of the pain it causes.
With a chilling smirk, Capricorn answered merely with a smile. "I'm sorry, Silvertongue, but the fact is I don't believe anyone. You ought to know that by now. We're all liars when it serves our purpose." (17.68)
Mo is claiming that he can't really control his ability to read people in or out of the books, but Capricorn doesn't believe him. The thing is though, that we know Mo is telling the truth here… and yet we've also seen Mo tell lies to people. This makes him a little hard to trust, and so (for once) we actually kinda agree with Capricorn: everyone lies when it's in their best interest, at least some of the time. Man, we sound really cynical, don't we?
"You're going back!"
"No, I'm not. I gave you my word. Have I ever broken it?"
Meggie shook her head. You broke your word to Dustfinger, she thought, but she didn't say so out loud. (27.7-9)
It sounds like Meggie is starting to have trouble trusting Mo now that she's seen him break a promise to Dustfinger.
"That really is an amazing story, old man," he said in a quiet voice. "I like it. You're a born liar, so I shall keep you here—for the time being—until I tire of your stories." (34.46)
Capricorn's words to Fenoglio are a little scary. He's basically saying that Fenoglio's lies amuse him, and so he intends to keep Fenoglio around as entertainment. We know, of course, that Fenoglio's not really lying, because he knows everything about Capricorn's backstory since he's its author. But for Capricorn to admit that Fenoglio's right would make him look vulnerable, so he can't do it. Lies upon lies—we're not really sure where to look for the truth anymore.
"What does Silvertongue look like? I think you've asked me before. Well, he isn't scarred like me." He tried to smile, but Resa remained grave. The candlelight flickered on her face. You know his face better than you know mine, thought Dustfinger, but I'm not going to say so. He's taken a whole world from me, why shouldn't I take his wife from him? (43.27)
Here Dustfinger's walking a fine line between lying and merely concealing the truth. He's figured out by now that Resa is Mo's wife Teresa, returned from Inkheart but left mute by the transition, but he doesn't tell Resa that Mo is the same guy as this Silvertongue that everyone is talking about.
"I buried them, and I'm certainly not saying where." Elinor felt a tear running down her nose. By all the letters of the alphabet, Elinor, she told herself, there's a great actress lost in you! (50.35)
Elinor lies to Capricorn, telling him that Mo and Farid are both dead because of the gunshots fired at them while they tried to infiltrate the village. We don't normally see Elinor lying in Inkheart, but she apparently pulls off a pretty convincing deception here.