The release date for the English version of 'Secret History' by
Brandon Sanderson is Jan 2016. If you enjoy this novel, it is available for buy as a paperback from Barnes & Noble or Indigo, as an ebook on the Amazon Kindle store, or as an audiobook on Audible.
Arcanum Unbounded: The Cosmere Collection also has it accessible.
The sequel to the first Mistborn trilogy is called Mistborn: Secret History. As a result, it has MAJOR SPOILERS for the novels The Hero of Ages, The Well of Ascension, and Mistborn (The Final Empire). Very slight spoilers for The Bands of Mourning are also included. Mistborn: Hidden History expands on the worldbuilding, characters, and plot of the first three books. Without that context, reading it will be, at worst, puzzling. Put simply, this isn't where you should begin your Mistborn adventure. (However, if you have already read the trilogy—even if it has been a while—you ought to be OK as long as you can recall the main characters and the overall storyline.) Any more words here run the danger of giving away too much. In a sense, the presence of this narrative itself is a spoiler. Another secret exists at all times.