He Likes Big Books
If you were going to a mental institution and you could bring only one book, what would you bring? We'd probably bring one of the ten longest books in human history, because we wouldn't be leaving anytime soon.
Karl has three books in the mental hospital. We imagine he acquired them during his stay, but we know they are the only possessions he takes with him he leaves, besides the clothes on his back. We'll let Karl give us the inventory.
KARL: One of 'em's a Bible, one of 'em's a book on Christmas, one of 'em's on how to be a carpenter.
If we didn't actually see three different books, we'd think he was talking about the same one. In fact, he's talking about the Bible, A Christmas Carol by Charles Dickens, and a helpful book on carpentry. But in a way, all of these books have Jesus in common. Christmas is Jesus' birthday. Jesus was a carpenter. And he's in the Bible somewhere, right?
What's with this common theme in Karl's little library? Is this because Karl has been studying the Bible, or is it because he fancies himself a savior, like Jesus?
Before murdering Doyle, Karl gives Frank his books and a "bookmarker." It says "You Will Be Happy." That adds to the Karl-as-savior analysis: he thinks he is sacrificing himself for the good of Frank.
Frank, take the books, and Jesus, take the wheel.