It's obvious, of course, but it bears repeating: words are the tools of a writer's trade. While the right word used correctly and at the right time won't save a book from its own crummy plot, unbelievable characters or lazy research, words used incorrectly can ruin a book that would otherwise be quite enjoyable.
I won't get into the nuts and bolts of good word choice and usage here, or anyplace else; there are plenty of books dedicated to all that. But I would like to share a few words that, while they aren't likely to be encountered in conversation or even in most writing, are nonetheless sparkly little gizmos of our language.

Librocubicularist: First of all, this one is really a bear to type. Second of all, I openly and shamelessly identify as a librocubicularist! I've been one since I was six years old, and I'm not alone!
Wait, put away your Awareness ribbons and hold off on that fundraising walkathon. A librocubicularist is simply a person who likes to read in bed. You knew there had to be a name for it, didn't you?

paraselene: Sounds vaguely chemical, doesn't it? Like it might be flammable, or good for healing irritated skin, or possibly be an essential nutrient for horses? Wrong. A paraselene is a "false moon" or rather, it's a reflected image of the moon in the sky, due to moisture in the air or some other unusual atmospheric condition. This is a rare instance where the scientific term is prettier than the common term.

Sesquipedalian: This one's a good example of a word that handily illustrates its own definition. Sesquipedalian literally means "having fifty feet" but in practice, it means a word that is unnecessarily long and complicated. The kind of words the insufferable Anne Shirley, red-haired scourge of Green Gables, liked to throw around to try and show off how smart she thought she was.

Quotidian: And here we see a word that is the opposite of its own definition. Quotidian is a pompous (sesquipedalian, perhaps?) word for "ordinary" or "everyday." Do yourself and everybody else a favor and never use this in speech, and only with great caution in writing. You will most likely look like a jerk even if (maybe even especially if!) your listeners know the meaning of the word.