Imperial Commando: 501st

Imperial Commando: 501st by Karen Traviss
The first book of an unfinished follow-up series to the beloved Republic Commando novels, 501st continues the story of those books into the early days of the Empire, when the clone commando squads are drafted into Darth Vader's special stormtrooper legion.

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This is a classic era book from the Legends timeline. It will likely be contradicted by newer books, movies, or TV shows (if so, just think of it as an alternate universe).