Book of Insight
From Cataclismo Official Wiki
The Book of Insight is accessible between campaign missions. In it you can spend Insight to improve units or the citadel, unlock new pieces, or give new passive or active abilities or Iris. Purchased skills can be refunded at any point. Certain skills require a higher Expedition Level before they can be unlocked.
Citadel
Citadel skills improve the citadel.
Units
Units skills improve the attacks of one the units.
Pieces
Pieces skills unlock new pieces.
Iris
Iris skills improve Iris' capabilities and unlike other skills they come in mutually-exclusive groups of two as well as two levels each.