Well, I'm glad I looked into this before publishing the book. It seems you can't include song lyrics in a novel unless you pay exorbitant rates to the copyright holders. If you include lyrics in your novel without paying the title holders, lawyers will swoop down on you and sue you for a zillion dollars. Apparently, you can mention a song's title but not one word of the actual lyrics. Oy.
This strikes me as weird and unproductive. Yes, I know people wrote those songs and they deserve something for their work -- but you're not copying the song and playing it on your TV show; you're just giving the tune some free advertising. And you're certainly not competing with the songwriter in any way. You're not re-selling his song, in other words. (Though I could see how someone might think differently about this.)
But at the core is something deeper: we're talking about Xmas carols here. They're as American as apple pie and "The Star-Spangled Banner". We were weaned on these songs. They are us and we are them. For this reason, it seems ridiculous to say you can't quote the lyrics of an Xmas carol in a book.
Anyway, it's a major bummer. I used the lyrics well in the book. They added something essential by connecting readers' Xmas memories to the story. I hate to lose that. Sigh. Now I have a huge hole to fill. Song titles ain't gonna do it. Bummer.