Sign up to be a part of different survey panels and wait for surveys to come to you via email. Taking online surveys for money typically means answering questions about products and services, but some panels will ask more interesting types of questions. Survey panels usually pay anywhere from $1 to $5 per survey taken or in the form of Amazon gift codes and other types of rewards. Here's a list of reputable survey panels to try:
Bookscouter will give you Amazon.com's trade in value, where you mail Amazon the book and get a gift card, they will not give you the used selling price because it's hard to compare apples to oranges. You may be able to get more for your book by selling it as a used book on Amazon, so be sure to double check that price. As an example, the trade in value of a book I looked up was $7.87 but used copies were on sale for $19.87.
Fiverr – Fiverr is a huge marketplace where you can sell all kinds of work. From products, like drawings and videos, to services, like a review of your website or resume. Fiverr's differentiator is everything costs just $5 at a baseline, though you can sell add-ons like faster delivery, etc; for more. The marketplace is huge and there's a lot of fun and creative things you can do.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.