Here’s a website that offers a lot of eBooks for sale, but also has a great section of free eBooks: Baen Free Library
Baen publishes only science fiction and fantasy titles and touts their offerings as “powerful plots with solid scientific and philosophical underpinnings are the sine qua non for consideration for science fiction submissions. As for fantasy, any magical system must be both rigorously coherent and integral to the plot, and overall the work must at least strive for originality.”
I’ve found several great science fiction eBooks on this website over the years – why not peruse it for yourself: you might find something you like? Once you click the link above, click on the “Free Library” link on the top left-hand side of the web page.
Once you click on one of the books and read its description and want it, you can download the Kindle version to your computer and then transfer it to your Kindle, Fire Tablet, or free Kindle reading app.
You can read them on your computer, or transfer them to your Kindle. If you download a Kindle book from the site, you will need to transfer it to your computer to your Kindle. That’s a fairly easy process. If you don’t know how to do that, you can click here or type in http://smarturl.it/xfer into your web browser to read my guide on how to transfer content to your computer. This is the same guide I charge 99 cents for in the Amazon Kindle store, but clicking the above link will allow you to read it for free.