You have two basic methods to load books onto your Kindle. First are the details on the tough method. Then a few sentences on the easy one. The difficult approach is to download the book to your computer and then transfer it by cable to the Kindle while it’s attached. You will never use this approach so long as you have access to wi-fi but you should know it.
The starting point is on Amazon’s website Contents and Devices page. You get there from the upper right of your Amazon home screen. The first tab shows everything you’ve already acquired that CAN but may not have been, downloaded to your Kindle. Anything there is available to you whenever you want. If you want something from there to show up on your Kindle, make sure your Kindle is hooked up to wi-fi and then tap the box to the left of the desired item and choose Deliver.
The second tab under Content and Devices is your Devices. It shows every device that has access to that Kindle library. This is where you can change the name of your Kindle to something cute if you want. It shows the model, the serial number, and you can turn off the ads for $20 (the same discount you received by accepting them in the first place).
The third tab is labeled Settings. It’s particularly useful if you want to email books or documents to your Kindle. You need to specify that you’re a person who is authorized to send items to your Kindle (near the bottom of the page). Also, in about the same location, is the email for your Kindle. You can literally email items ending in *.doc to your Kindle that way.
However, my favorite way of getting non-Amazon items on my Kindle is to use a “Send to Kindle” app available at https://smile.amazon.com/gp/sendtokindle. Amazon has them for any type of computer you have. You install it on your desktop in your dock or on your desktop — wherever you can find it easily later. Now, when you have something you want to deliver to your Kindle, you drag and drop it onto the app. It will ask which Kindle(s) registered to your Amazon account that you want to send it to and whether you want to add it to your archives (the answer is always yes). Drag, drop, say yes, you’re done. No cables. Easy.