I've tried to keep this one spoiler-free, but really, you should buy the issue of Speculative Mystery Iconoclast and read it first - just to be safe!
The idea for Mirror, Mirror ... came during a period when I wanted to write a pseudo-Victorian piece. I don't remember what sparked this, but I purchased the Writers' Digest "Everyday Life In ..." book on the topic. The result of this research ...
... was not Mirror, Mirror .... At least, not at first.
It was another story, entitled The Changeling Letter, which is still looking for publication. In working on this tale, I came up with a setting that would later be reused in M,M: the kingdom of Gloriann. (I also used Gloriann and its neighbor, Tarmaria, in a story set at the beginning of the queen's rein. It's also a mystery story; however, I retired it because I just couldn't find a way to cut down the complexity or the cast without compromising the story. Editors thought it was too much; they were right.)
In writing TCL, I had a weird out-of-body sensation. Lines came out of me where I had no ideas of their origin. It wasn't my style, but it flowed perfectly. This was to happen again as I wrote M,M.
In doing my research for TCL, I made notes on some things that interested me. Two that applied directly to the story was the idea that people used to cover mirrors while in mourning, for fear the soul would otherwise become trapped inside it; and the concept of the first interior decorators. I also was tickled to death by descriptions of period bathrooms, so one appears in M,M as well.
These concepts collided to form the concept of a murder mystery where the consequences of death were so much more than merely departing from this life. I knew I wanted to use the same setting, but I wanted a sort of psychic detective, so I had to set the story later than TCL, where magic is forbidden to women. Things have loosened up a little in M,M, but my main character still has to pretend to use devices to guise her own powers.
I chose the name Graeme because I saw it on a book cover (GURPS Faerie, actually) and thought it was fantastic. Much later, I learned that it's probably a Celtic variant of Graham, but that's all right - the awkward "outsider" moment my narrator has with another female character in the story makes the choice of a masculine name apropos.