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This is the only book in this guide where I wouldn't recommend the other installments in the series. Most of them follow what happened in the game, and once you've played it, you certainly don't want to read that all again.
Forsaken is different, as it follows Haytham Kenway (Connor's father), and you begin to understand how he became a Templar and what drove him to make those decisions. It's a fantastic book for shedding light on an otherwise unknown character. Plus, it is perfect for any hardcore fans of Assassin's Creed.