I distinctly remember one of the novels implying that the "slingshot effect" was something only Starfleet knew about, and that it was secret. Kirk knew about it, of course, because he utilized it both by accident and on purpose for a Starfleet assignment in TOS. But because pinpoint accuracy is required, and the maneuver extremely hazardous, it wasn't a reliable means of time travel.
You're probably thinking of my recent novel Department of Temporal Investigations: Forgotten History
. It explains why the slingshot effect is a dangerous maneuver and one that not all ships are capable of achieving. There is precedent for this; "Tomorrow is Yesterday" showed that the ship was pretty banged up by the slingshot and barely came through it intact. "Assignment: Earth" didn't show the actual slingshot in either direction, so it could've been equally hazardous. TVH didn't show it as being quite so turbulent, but it wasn't easy either; Spock had to get the computations just right to pull it off.