Harmony is the name of the planet in a far future ficton where Mike Callahan, Lady Sally McGee, and Mary Callahan-Finn originate from. Few details are known about it other than the fact that the natives have somehow mastered time travel and have a nearly perfected system for predicting future events from any given point in time, based largely on an understanding of its available technology. Most stories seem to imply that Harmony is simply a new name for a far-future earth where, Mike Callahan asserts, "humanity has it together."
(One detail mentioned in a Lady Sally story is that the population of the world has increased to over a quadrillion, and most humans of that ficton now live for some centuries. This serves as a strong Harmonian incentive to ensure that their future is allowed to come about--a by no means certain thing, given the overall state of the world.)
Maureen Hooker is the only modern-day human known to have been to the Callahan's ficton during the events of Callahan's Lady. Most of what she could perceive was insubstantial. This theme of insubstantial "furniture" is continued in the sequel, Lady Slings the Booze, where it is found in the basement workplace of one Nikola Tesla, an historical inventor who now works for the Lady; he has not been directly shown, however, to have gone through Sally's portal into her ficton. (He does appear, though, to have been resurrected and artificially "youthened" by Harmonian technology, having died historically in 1943, at the age of 86.)