Mistborn: Secret History is a companion story to the original Mistborn Trilogy.
As such, it contains HUGE SPOILERS for the books Mistborn (The Final Empire), The Well of Ascension, and The Hero of Ages. It also contains very minor spoilers for the book The Bands of Mourning.
Mistborn: Secret History builds upon the characterization, events, and worldbuilding of the original trilogy. Reading it without that background will be a confusing process at best.
In short, this isn’t the place to start your journey into Mistborn. (Though if you have read the trilogy—but it has been a while—you should be just fine, so long as you remember the characters and the general plot of the books.)
Saying anything more here risks revealing too much. Even knowledge of this story’s existence is, in a way, a spoiler.
There’s always another secret.
Wow, just WOW!
Why aren’t you already reading it?!?
Neat, but not great
A fun, quick romp with Kelsier through a world much bigger than anyone from Scadrial might have expected. Still, this isn’t Sanderson’s best. It’s fun, and runs concurrently to the last two books of the Mistborn trilogy with a few little explanations over some of the relatively unexplained, minor events. Read this if you love Mistborn, not if you’re looking for a great story. It’s a companion novel, filled with Cosmere trivia, and for that I enjoyed it. But it doesn’t have much of a storyline of its own — no real character arcs, no twists. If you missed Kelsier, though, this should help!
This book...oh my god
Definitely don't read this until you've read all of the mistborn books up to Bands of Mourning. This little novella will make you feel nostalgic in the best, worst, possible way.