This is a charming book. Funny, interesting, with a really creative premise. There's even a cute twist at the end. I've been thinking about it for days, that's how charming it was. But then I looked at its page on Amazon.
Some people on the internet, it turns out, are crazy people. I suppose in any sample of the population there will be people who haven't paid very much attention to American history. And I suppose there will also be people who look at Richard Nixon's face and think they're looking at a groin. But just because a reaction like this is possible to have does not make it reasonable to have.
So here's the deal. If you don't recognize the person on the last page, you should have paid more attention in history class. That's your fault. Don't take it out on this excellent little book. If you recognized the person on the last page as Richard Nixon and understood that he was a less well-reputed president than Abraham Lincoln, and by implication less enviable to look like, and still thought the reference to "Little Dickie" was an inappropriate joke, then that's something you need to work out with your therapist.
And if you are a parent or a legal guardian and your child laughs at the end because of the word "Dickie," use the opportunity to teach the little bugger about Watergate and explain why it's actually funny.