Tracy Giacomo was a 23rd century Human woman. She was an ensign who served in Starfleet in the sciences division aboard the Federation starship USS Enterprise under the command of Captain James T. Kirk in the 2260s.
In 2268, Giacomo worked a night shift in the central computer room. While providing routine input to the core, she felt a chilly presence and heard non-directional knocking, which spooked her into thinking the place was haunted. The noises actually were produced by a disembodied James T. Kirk, who was attempting to communicate with her. Afterward, she began working double shifts to keep others on duty company, in the wake of similar unnerving experiences aboard the ship. Later, she shared coffee with Dan Miller in the core room. (TOS novel: Ghost-Walker)
In 2270, when the Enterprise was captured by the crew of the time travel vessel Nautilus, Giacomo was among those trapped in the powerless deck 11 recreation lounge, unable to access anything stored behind magnetic locks. She suggested using vegetable oil to make candles, which prompted Miller and John Brunowski to trick the food synthesizer into producing grease. She, Jefferson and Emiko Adams unwound needlework projects to create the wicks. (TOS novel: Crossroad)