It's been a while, but wasn't there a mirror mirror story line in the Shatner-verse novels too?
Yes, in the novels
Spectre, Dark Victory, and
Preserver. That's one of several 24th-century Mirror Universe followups that I didn't mention because they weren't in the immediate aftermath of "Mirror, Mirror" and weren't in continuity with
The Sorrows of Empire. Other takes on the 24th-century MU include TNG:
Dark Mirror by Diane Duane (which predates DS9: "Crossover" and thus depicts an intact Terran Empire in the 24th century) and the duology
Dark Passions by Susan Wright (based on the DS9 Alliance version of the MU).
Also in comics there's the 1995 story "Enemies and Allies," a backup story in issues 29-30 of Malibu's DS9 comic co-written by Tim Russ and featuring the Mirror Tuvok; and the 2008 IDW miniseries
Mirror Images, which is mostly a prequel to "Mirror, Mirror" but includes an odd and rather pointless diversion to show Mirror Picard on the
Stargazer, interpreting the fall of the Empire rather later than other sources do.
The novel
Stargazer: Three features a character who comes from a version of the Mirror Universe, but it doesn't actually go there. And DS9:
Millennium: War of the Prophets references events in an alternate future that involved a crossing to the MU, but it's a secondhand account.
And I think that's a comprehensive list. Not counting video games.