Miss Marple is actually actively present for this whole novel!
Seeing that it's set in a philanthropic reform school for JD boys, shows off Christie's classism hangups, but I see this in every one of her books I've read. Putting that aside, I thought this was a fascinatingly peculiar piece of fiction to be a work of popular entertainment. A truly odd story. I could care less about figuring out who dunnit; these things are all contrived, so just about all mystery fiction fails as "puzzle" entertainment to me. But the interviews of the characters with the investigating detective, in which they are all blase with someone shooting a loaded gun at someone they know, and the investigator's growing frustration and confusion at their dismissive reaction in this regard, got to be very amusing.
Fun, weird characters. In a way, many of these Christie novels feel grotesquely Goth tinged ; sometimes reading one can feel like watching the Adams family. or an Italian Giallo horror movie, without the gore and luridness. ( )

Ending is a bit too ridiculous and the book in general is too full of the stupid prejudices of the upper middle class for me to forgive it

So the setting is a manor house converted into a rehabilitation centre for young petty criminals, where they try and work through their trauma problems and get them placed in proper jobs. Multiple characters get mad at the very concept, (as well as state education - it's a billionaire's charitable education trust that got retooled) talking about the boys in nasty terms for no reason and saying it's a waste of money while praising people who magically pulled themselves up by their bootstraps. Mrs Marple's screed against it gives the game away a bit by emphasising support should be given to people from "good families" lol, there's quite a bit about heredity in this although mostly it's not borne out in the denouement. When we see the boys a few times they're totally fine. But unsurprisingly the ending has the guy running the centre be involved in a complex embezzling scheme where he trains the boys he places in financial and accounting jobs to send him money which he's putting aside for some bizarre colonial scheme where he thinks criminals should be transported like to Australia so they can start again it's such a ridiculous motive!

The writing is also a bit weaker than usual - lots of explication is done through verbatim police interviews. The ending is a bit silly so the son has been pretending to be delusional for however many months just in case they need him for an alibi? Rough but it's mostly well hinted at as per usual. ( )

Enjoyable, as I have come to expect from these stories. I had started to see the whodunnit on this one, but got distracted and ended up chasing a red herring. ( )

Now You See It
Review of the William Morrow Paperbacks edition (2009) of the Collins Crime Club (UK) & Dodd, Mead & Company (US) hardcover (1952) originals.

“When you only look at one side of a thing, you only see one side,” continued Miss Marple. “But everything fits in perfectly well if you can only make up your mind what is reality and what is illusion.”

Miss Marple is asked by an old friend to look in on her sister Carrie-Louise who lives on an estate where her husband administers a reformatory for delinquent boys. There is suspicion that Carrie-Louise is being poisoned, but then someone else is murdered by gunshot instead. Further poisoning attempts seem to occur but again others are murdered by other means. The house is separate from the reformatory so the delinquents do not appear to be the culprits. Only Miss Marple can see through the trickery to arrive at a solution where the police are baffled.

See cover at https://upload.wikimedia.org/wikipedia/en/e/e6/They_Do_it_With_Mirrors_First_Edi...
The front cover of the original 1952 Collins Crime Club (UK) hardcover edition. Image sourced from Wikipedia. It is believed that the cover art can or could be obtained from Dodd, Mead and Company., Fair use, Link.

This was another wonderful cozy with Miss Marple proving her mettle against a devious culprit. I am continuing to enjoy these Agatha Christie classics in a binge-read in between my other books.

Confusion for Completists
They Do It With Mirrors is the 5th Miss Marple novel. Some lists, including the Goodreads Miss Marple Listopia, count it as Miss Marple #6 as the short story collection The Thirteen Problems (1932) is counted as #1.

Trivia and Links
They Do It With Mirrors was adapted once for an English language TV movie and twice for English language television series. I have not seen the film, but both of the TV adaptations are reasonably faithful to the original plot. I did not find any free trailers or postings of either of them, but they are both available on the Britbox streaming service here in Canada.

The English language TV movie adaptation was Murder with Mirrors (1985) with Helen Hayes as Miss Marple.

The first TV series adaptation was as part of the BBC's Miss Marple (1984-1992) series as Episode 11 in 1991, which starred Joan Hickson as Miss Marple.

The second TV series adaptation was as part of ITV's Agatha Christie’s Marple (2004-2013) reboot series as Series 4 Episode 3 in 2010 which starred Julia McKenzie as Miss Marple.

There was a French language TV series adaptation for the Les petits meurtres d'Agatha Christie [French: The Little Murders of Agatha Christie] (2009 - ongoing) series. The episode based on They Do It With Mirrors was Season 2 Episode 1 Jeux de glaces (French: Game of Mirrors) (2013). This series does not feature a Miss Marple character and instead has a police detective and a reporter as the leads. The plots are transplanted to France and are considerably changed from the originals. ( )

I felt in the mood for something light and quick, so I turned to an Agatha Christie mystery. It was enjoyable, with a fun cast of characters. I find these predictable, but great comfort reading. This was a Miss Marple, and I think I like hers best. ( )