Book rights, and especially eBook rights, can be messy. UK publishers can't just start selling books in the US, and vice versa. For most modern books by big authors, you'll see the hardback and eBook versions being published at pretty much the same time around the world. For older books that were released before eBooks were a big deal, and for smaller authors with publishing deals, however, you'll regularly find that the eBook version is available in some countries and not others.
Check out this book, The King Arthur Trilogy Book One: Dragon's Child by M. K. Hume. For me, it's available on Kindle in the UK Amazon store.
...But not in the US one.
If you're an American fan of Hume, this would be pretty annoying, since you can't purchase books from the UK Kindle Store. I run into this issue every so often, but fortunately, there's a workaround: it's possible to change which Store your Kindle uses.
For this to work, you need an address in the other country. For example, if you're American and want to use the UK Kindle Store, you need a UK address. There are two simple ways to get one: ask a friend who lives in the UK if you can use their address for Kindle purchases, or just make one up. For me, who needs a fake address in the US, I'm partial to the ZIP 90210 myself. As long as you're not getting physical products delivered, Amazon won't notice that 123 Harry Street, Beverly Hills, California, doesn't exist.
Whatever way you get your address, the process is the same. Open the Amazon version of the country you're currently using. For me right now, that's Amazon.com.
Hover over the Account and Lists dropdown and select Your Content and Devices.
Next, select Settings.
Then, under Country Settings, click Change.
Enter the address you want to use in the country you want to change to and click Update.
And just like that, Amazon thinks you're a resident somewhere else. You'll then be able to purchase items from that country's Kindle Store.
I've done this a few times a year, as I've run into books not available in one Kindle Store or the other, and never run into any problems. It's never interfered with also buying products from Amazon, even if they think I'm resident in the US when I'm in Ireland. If you do encounter any issues, just swap your resident address back to your real one, using the same process above.
Looking for other handy Kindle features and tricks? Learn how to keep Kindle library books past their due date.