Note that it's not uncommon for books to be mislabeled in Google Books, so you'll find some instances of authors apparently time traveling to write about new inventions years before they existed! Big picture wise, though, these graphs look pretty good.

Changing media types. Hello, radio in WWII era books! And look at the tiny little Internet, just starting to get in the game, there in the right hand corner.

Seven deadly sins. Looks like we're growing less fixated and finding other things to write about.

Angels vs. Unicorns vs. Mermaids. Not even close.

Garlic vs. Onion. We are living in changing times, people.

Carrot vs. Celery Celery probably thought it was pulling into the lead for good. Not so!

Major religions of the world. Indexed literature is growing increasingly diverse.

Drugs

Phobias and prejudices. Is this a chart of what the world cares about most?

Nerds, Dorks, Geeks and More

Authors: Huxley vs Camus vs Leary vs McLuhan Honestly, I think this one is the one that surprised me the most. If not carrot vs celery.

Want to see more? There's a whole community of Ngram collection going on over at Ngrams.Tumblr.com.