Any books that use mirrors in this way?

Well, Harry Potter sort of uses this concept. There are two mirrors specifically that you need, it's not just any mirror available. And Harry hides the mirror away and forgets about it and because he forgets about this form of communication, goes to the end of the world to try and save Sirius who dies, anyway. And later breaks the mirror, but he has a piece left and he sees "a flash of blue" in Deathly Hallows and thinks it's Dumbledore. And so on.

But the whole idea of using reflective surfaces (water, glass...) for communication is pretty common, in general. Someone else mentioned the orbs in Lord of the Rings. And there's the "Mirror, mirror, on the wall, who is the fairest of them all?" thing from Snow White. I don't think you would be copying from any one source by using this idea, though, perhaps you may want to tweak it a bit?

Woah.

So you could stumble across a mirror in your story that's showing the reflection of another place? That would be so creepy, especially if like, someone is getting murdered in the other place, what would you do? I'd probably run at it thinking it was some sort of portal, breaking it, and losing the reflection. You could totally make a murder mystery out of that...

I thought of Harry Potter too, but like you said, the plot didn't revolve around it. Don't worry about where reflections have been used, it's going to be unique to your story.. unless you take their plot too...

Are you familiar with "Nine Princes In Amber" that was quite popular in sf/f circles some years ago? The devices weren't mirrors, but playing cards -- of all things -- but the connection idea was a bit similar.

Good luck with the new book -- sounds fascinating!

I don't think they are a major part of the plot, but I remember mirrors being utilized, mostly for travel I believe, in the novel "Jonathan Strange & Mr Norrell" by Susanna Clarke, which came out a few years ago. The novel is set in early 19th century England and involves two magicians, one a mentor to the other, who become adversaries.

Cleaning and Laundry? Wtf??

Um, the only thing I can think of that uses mirrors like that is the horror film called Mirrors... although I don't think they have to be in pairs. You'd have to watch it to get what I mean. I'm not sure if it's what you meant or not.

But yeah, sounds like an awesome idea :)

I honestly can't think of any book I've read or heard of that uses mirrors of that kind (and I did have a good think about it). Most of the time it tends to be doors of one form or another, even mirrors that act like doors, but that's different of course.

I think even if there are books out there that use that idea, and despite the limited scope for uses of such items, the chances of you unintentionally reproducing parts of those books in a similar fashion are incredibly slim.

There's one that just came out by the author of Inkheart. Slightly different twist (it's more like a portal), but there's also Snow White.

Closest thing I can think of is those glass orb things in Lord of the Rings.