Mistborn: Secret History by Brandon Sanderson. ★★★★
Misborn: Secret History is a surprise novella released alongside The Bands of Mourning. It contains huge, massive, epic spoilers for the original Mistborn trilogy! DO NOT READ THIS NOVELLA IF YOU HAVEN’T READ THE ORIGINAL MISTBORN NOVELS! Seriously, don’t do it! In fact, you probably shouldn’t even read it until after you’ve read The Bands of Mourning (this is what Sanderson recommends).
Also, Mistborn: Secret History is basically cosmere backstory. So it’s probably best that you have some knowledge of the cosmere going in. The cosmere is the universe that contains Sanderson’s adult epic fantasies (namely, The Way of Kings, Mistborn, Warbreaker, Elantris, and some yet unreleased novels). These series are all set in different worlds, but the worlds are partially unified. They have some of the same underlying theory of magic that depends on three realms, Physical, Cognitive, and Spiritual. Part of Words of Radiance is set in the Cognitive realm, and almost all of Mistborn: Secret History takes place in it.
There are a few characters who are able to travel between realms. The most obvious is Hoid, a mysterious character who’s appeared in every cosmere novel. He’s most prominent as Wit from The Way of Kings and Words of Radiance. There’s a group of people the fans call the Seventeenth Shard who also travel between realms. Shards are another important cosmere concept that relates to the magic system.
Honestly, if any of this is news to you, you probably shouldn’t be reading Mistborn: Secret History. I’d suggest reading some of the other cosmere novels first and checking out the online forums to get up to date on the fan collected knowledge.
As for talking about the plot of Misborn: Secret History itself… I cannot be at all specific if I want to avoid spoilers. It’s basically a “behind the scenes” of the original Mistborn trilogy. It’s interesting to see how there was all this stuff going on that we didn’t know about and that Sanderson’s been planning since 2004! I also liked being back with certain characters from the original trilogy, although in her review, Anila does make some good points about how this effects characterization (especially regarding Vin).
If you have an interest in the cosmere (or a huge love of the original Mistborn trilogy), Mistborn: Secret History is definitely something you should read.