Star Trek: Section 31

Section 31 was a series of four novels, each centering on the activities of Section 31, the secret covert operations division of Starfleet. (DS9 episode "Inquisition")

The titles in this series are:
 * Cloak (TOS)
 * Rogue (TNG)
 * Abyss (DS9)
 * Shadow (VOY)