An Amazon Kindle is a lot like a laptop, or a cell phone, or a house: Once you buy one, you have to buy things to fill it up to make it useful. An empty Kindle is a fairly useless hunk of tech.

Filling your library up with good reads can cost a small fortune, if you're not careful. Luckily there's a way to meet your reading goals without breaking the bank — you just have to know where to look.

Amazon has sold tens of millionsof Kindle e-readers, a remarkable feat even for America's largest retailer. But if you don't already own one, you can avoid buying a Kindle altogether by downloading the free Kindle app onto your iPhone or Android.

Now, on to finding some free ebooks.

Many of the ebooks you'll be able to download for free will likely be classics or books that are already available in the public domain, but there are ways to find newer books to download to your Kindle for free, too.

Here are the four easiest ways to load up your Kindle library without unloading your wallet.

1. Search the Kindle bookstore

On your Kindle or on Amazon.com, search "free kindle books." It really is that simple. Lists will pop up, and you can navigate around a bunch of free books available for download. A lot of these books are smut, but they're free smut!

Amazon also has a list of the Top 100 free ebooks, which currently includes all of Marc Twain's novels, The 7 Habits of Highly Effective People: Powerful Lessons in Personal Change, and The Handmaid's Tale, among others.

2. Get an Amazon Prime or Kindle Unlimited subscription

Alright, so this isn't technically "free" in that you do have to pay $9.99 per month for the unlimited subscription. But, with that subscription, you'll get loads of books.

And if you really want to be explicit with the definition of "free," you can always set up a free one-month subscription and try it out to see if it's worth that $9.99 price tag. Just remember to cancel before time's up if it's not working for you.

3. Download free ebooks from your local library

You can get some downloadable ebooks through your local library, or an online library like Libby, which just requires that you have a library card. This is a great way to get some newer releases.

It works similarly to checking out an actual book, in that libraries have a specific number of copies to lend. If all copies are already checked out, you'll have to wait your turn — and you can't hold onto the ebook forever.

4. Check out your options for downloading free ebooks on other apps

There are loads of resources outside of Amazon that can set you up with free ebooks, like Project Gutenberg, which is a collection of public domain books that are available in ebook form and are compatible with Kindles to download or read online. BookBubis similar, but stems from an email subscription service that alerts you when your favorite genres release free versions of books, or books at a discount. Smashwordshas a catalog of free ebooks, many of them self-published. You can also sign up for the Kindle Daily Dealsnewsletter, which flags free and cheap ebooks. Finally, there's eReaderIQ, which tracks ebook store price changes and offers an evolving selection of free and heavily discounted books.