Kindle Unlimited is a service offered by Amazon.com. For a monthly subscription fee, you can read or listen to as many online books as your heart desires. The collection has over one million book titles in print, and thousands of audio books. Also, it has a substantial collection of magazines. If you don't have a Kindle device per se, don't fret. You can download the free Kindle application to another variety of tablet, smartphone, or computer.
Google Books is a combination digital library and searching service available from — you guessed it — Google. You can go directly to Google Books and enter a search term, then scan the results. Or you can log into your Google account (associated with your gmail account if you have one), then start the search process. When you enter the search terms and a list of results generates, it will include out-of-print books that are available for viewing on Google, as well as books that you can purchase or check-out/borrow printed copies.