(written from a Production point of view)
Star Trek: Mirror Universe is a novel series published by Pocket Books. The first two comprise collections of three novellas, the third a series of short stories, and the fourth is a novel expansion of one of the earlier novellas.
The DS9 novel Fearful Symmetry was originally intended to form part of this series, as well as being the latest edition of the novel series set after "What You Leave Behind". However, the novel as published does not feature the Mirror Universe title. The Soul Key is also set during the time period.