Inkheart is the title of our book… and the book within the book. Gah.
Inside our story there's a dude named Fenoglio, who wrote a book that he titled Inkheart because, as he explains it, "Its title is Inkheart because it's about a man whose wicked heart is as black as ink, filled with darkness and evil" (34.15). Since this man—a.k.a. Capricorn—got read out of Fenoglio's book and into the world of the book we're reading, we've gotten to know him a bit by now, and we completely agree that he fits that description.
In a broader sense, though, it makes sense for the book (the one we're reading) to be called Inkheart because it refers to the book that sucked in Meggie's mom and spat out Capricorn, Basta, and Dustfinger, all of whom are key characters. This development upset Meggie's life, impacting the life she and Mo lead in many ways. We're guessing that it hasn't been fun to flee like refugees every couple of years in order to try to escape Capricorn's attention—and yet that's exactly what they've been doing, though Meggie didn't understand as much until Dustfinger shows up on their front door.
The title is also kind of play on words, since ink—in the sense of the printed word—is central to the plot. Both Mo and Meggie have the ability to read written words out loud and magically manifest characters and items from those stories. Which is pretty nifty, huh?
The book Inkheart (the book inside our book) is also a focal point of the plot, because pretty much everyone wants to get their hands on it: Capricorn wants to burn all the remaining copies but one so that he can't be sent back into the book's world; Dustfinger wants a copy so he can be sent back in; Mo wants a copy so he can try to get his wife out; Basta wants a copy because Capricorn wants a copy; and Elinor wants a copy because it's a rare book and she wants to read it. Whew—that's a lot of wanting for just one book.
And those are a lot of associations for one title to convey, but we think Inkheart manages to pull it off quite nicely.