By pure chance I found two decent eBook readers for Windows, that are free for a limited time. There is the eBook Reader for Kindle Books, MOBI, EPUB and more and the Pro version which also allows to download free Kindle books. They are essentially the same product and usually sell for 10 and 20 EUR before discounts. If you are reasonably fast, you might be able to grab a free copy.
Unlike the Kindle Reader for Windows, you can sideload books. You just need the book on your file system and can open them right in your app. I also like the fact that there is not much chrome. It’s just the book and little else. You can set the colors and fonts to your liking, and that’s all I need from a book reader.