Mine's got to be a tie between Darth Bane: Path of Destruction, and Coruscant Nights III : Patterns of Force.
Love the hardcore factor of Darth Bane and a look into the life of a Sith (we see jedi's so often).
The Coruscant Nights trilogy (will be a quadrilogy eventually) opened up a new dimension to the force, the way it describes the character's awareness of the force in a different way for example Jax Pavan saw the force as ribbons of color.
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