This is a fun type of question. It's directed in that you have to stick with songs, but it's wide open in that it allows for a person to pick songs throughout all genres and time periods. To answer this question, consider a character's traits or the actions a character takes in the novel and choose a song that highlights that character's traits and/or actions.

For me, one song immediately comes to mind when thinking of Meg Murry and her actions in the story. Charles Wallace is absorbed into IT in the story, and it is Meg's love that brings him back. The song that comes to mind is "The Power of Love" by Huey Lewis and the News. The song talks about love in general ways and focuses on love being a powerful and unstoppable force that is "tougher than diamonds."

All 4 One's "I Swear" could work for Meg, Calvin, or Charles. The three characters are strongly devoted to each other and helping each other out of any and all obstacles that they encounter. The song is focused on romantic love, but the lyrics do highlight the strength of a bond between people.

And I swear by the moon and the stars in the sky
I'll be there
I swear like a shadow that's by your side
I'll be there

When the road looks rough ahead
And you're miles and miles from your nice warm bed
You just remember what your old pal said
Boy, you've got a friend in me
Yeah, you've got a friend in me