Darth Plagueis by James Luceno
A story set over the course of decades, this is the history how the Sith rose again to power through the schemes of Darth Plagueis, the ambitious Sith Lord who trained the later-emperor Palpatine in the ways of the Force!
In many ways, it's more of a biography of Palpatine (a.k.a. Darth Plagueis) than of Plagueis himself, so it comes very much recommended for anyone interested in young Palpatine as well.
A fan-favorite novel, and something of a masterpiece of the late Legends publishing days, pulling together and drawing dots between many of the other Legends books, comics, and even video games set during the prequel era.
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This is a prequel era book from the Legends timeline. It may be contradicted by newer books, movies, or TV shows (if so, just think of it as an alternate universe).