Go ahead, tell people about your book.
Here's a true story with a couple of morals.
The other day, I popped around the corner to the grocery store to pick up a few things. I was in sweatpants (what? I am a writer) and my Hunger Games t-shirt. At the cash register, the cashier took one look at my shirt and said, "Oh my gosh. Did you read the books?"
We had a great talk about our favorite books of the trilogy, our thoughts on the end, and how the film did as an adaptation. After I swiped my card for the second time (we were too busy gushing about Mockingjay to hit the right buttons on the computer/card machine) and I was about to leave, I said, "I'm going to be super awkward for a minute, but since I know you like to read, here's my card." And I gave her my business card with the Incarnate cover on the front and my contact information on the back.
She took one look at the cover and said, "That looks good." That's how normal people choose books. That's not (necessarily) a bad thing.
Moral the First: Book covers are meant to attract the right kind of reader for what's inside. It doesn't always work, but they're meant to communicate the genre, mood, and a ton of other details I'm not smart enough to know about. They're supposed to make people say, "That looks good" because what those words really mean is, "That looks like a book I would be interested in reading."
If you can, put your book cover on your card to make it a good reminder and way to find your book in the store.
Then the cashier asked me what the book is about, as she checked out my contact information on the back. I gave her the short pitch that was on Publisher's Marketplace (hint to writers: memorize your PM line; it's useful) and she still looked intrigued. She promised to look for the book and email me if she liked it. Which brings me to...
Moral the Second: Don't be afraid to give out your card. (Keep your cards with you. Always.) I find this works best if you've already connected with someone about a book or something you both like (as Hunger Games) or they've asked what you do for a living, because random card-hurling is just weird. But the truth is that, for most of us, people aren't going to know you write books and have a book they can buy unless you tell them.
It takes courage. I've let opportunities slip by before, out of fear, and I always regretted it later. But if you're friendly, polite,[1. Don't just assume they're interested. Pay attention to their body language and how they're speaking. If it seems like they want to get away, stop talking about your book and let them get away. If someone wants to talk to a sales person, they'll go to a car dealership. Let them decide how far to take the about-your-book talk.] and prepared for the most common questions ("what is it about?"), most people are pleased for your success. Most people are delighted to have made a personal connection with an author. I hear a lot, "I've never met an author before!"
And who knows? You might just introduce someone to their next favorite book.[2. A girl can dream.]
Bonus Moral: Wear your Hunger Games t-shirts everywhere.