The Starfleet Handbook on Personal Relationships was a guide to Starfleet officers and crew on how to deal with personal relationships, with alien species and presumably within those of the same species. It was three centimeters thick.
In 2375, Harry Kim quoted a portion of it to Captain Kathryn Janeway: "All Starfleet personnel must obtain authorization from their CO as well as clearance from their medical officer before initiating an intimate relationship with an alien species." (VOY: "The Disease")