Drive

This is a tragic short story about an invention that enables humanity to colonize the solar system. Mars is already settled, and we see some plausible differences in culture between humans on Earth and Mars, which eventually lead to war (the backdrop of the first novels in the series). Apart from giving a historical background to the rest of the series, the plot itself stands alone. It's short and bittersweet, and very good. Originally published as a part of the science-fiction anthology Edge of Infinity, and recently made available online for free to promote the upcoming The Expanse TV series.

Cheating a bit by including the auxillary short stories and novellas from The Expanse, to make up for counting all of LoTR as one book. Gotta meet them quotas.

I wouldn't even call this a prequel as much as background knowledge, but this was a great story. It isn't necessary in order to appreciate the main books, so it feels like finding an easter egg in a video game.