When it comes to Superman, Mark Millar is well known for the Elseworlds series Red Son, featuring a Superman who landed in the USSR rather than the USA, which has been adapted and spun off in all manner of forms. And for his run on Superman Adventures, as a young writer, an all-ages series that contains what some consider to be the finest Superman stories ever told, including one famous example that had 22 complete stories in one issue.
He was also part of the Hive projects, with Grant Morrison, Mark Waid, and Tom Peyer, asked by one editor to pitch how they would take over the Superman comics as a group, only for the Superman group to take umbrage at the pitch and see Waid's copybook on Superman at DC marked for all of Paul Levitz's time.
Mark Millar has also written versions of Superman as Apollo, Superior (with a bit of Shazamness) and in Jupiter's Legacy. But he wrote a number of other Superman comic books, that haven't lasted or stood out quite as much. But that's okay, because DC Comics is going to collect them anyway.
Superman by Mark Millar, to be published by DC Comics in June next year, collects Mark Millar-penned stories from the following,
Adventures Of Superman #573-576 and Action Comics #753-755 & 758, plotted by Stuart Immonen but only scripted by Mark Millar.
Superman for the Animals #1, a giveaway comic from around 2000, that doesn't actually have a lot of Superman in it — apart from him reading letters about animal cruelty. Then there are one-shot stories, Team Superman #1 with a Superman who has gone missing, a short story from Superman 80-page Giant #2 focused on Lois, with Sean Phillips and possibly the highlight of the book, and Millar's contribution to Tangent Comics: Superman #1, which gave a us a very different take on what a Superman could be in another reality.
So weirdly, for a collection entitled Superman By Mark Millar, half of it only has Mark Millar scripting over someone else's plots, and the other half have Superman comics that the actual Superman is hardly in.
Maybe Not Entirely Superman By Not Entirely Mark Millar would have made for a better title? Or even Scraping The Bottom Of The Superbarrel? Hey, if Mark Millar won't write Superman for DC Comics anymore, they have to do something…
Millar's far superior Superman Adventures and Red Son do, however, remain in print.