Far more interesting would be to ask:
What novel (or series, movie, TV show, etc.) would you actually want to be in? You don't get to be one of the main characters. You can choose to be a (new) minor supporting character, or just to live in the world described.
It would be more true to the original meme to ask which Heinlein novel you'd want to live in, but frankly, it's been so long since I've read most Heinlein that I wouldn't feel ready to answer it. So I expand the question to one that at least intrigues me.
The more I thought about this, the more books I could think of that I'd like to live in. But rather than spend a lot of time trying to think of the one book I'd most want to live in, I will just give the first answer that came to mind when I thought of the question.
I would like to live in the Blood series. I'd want to be a friend of one of the vampires -- trusted with their secret. Not the new love of their life, just a friendly person to talk to (and a willing meal) from time to time. I don't want to be dragged into the adventures (not unless I get abilities I don't have in this world that would make me more than a hapless target).
Feel free to answer for yourself in your own journal. Deconstruct the idea or my answer here.