The character Montmorency was originally a thief and the series begins when he suffers injuries while being chased by the police. He is pretty banged up, but a doctor (Dr. Fawcett) puts him back together and then hauls him around to lectures in order to show off his handiwork. Updale has an uncanny ability to capture the feel and attitudes of the time period (nineteenth century England/Europe). She makes it clear that Fawcett perceives Montmorency, at this point, as little more than a speciman. There is no friendship between the men, and they don't meet again until after Montmorency has become Montmorency.
This understanding of class and class attitude continues in the other books. Although Montmorency's friends in general know his past, they are appalled by his low origins and practice great forebearance by overlooking them. Montmorency himself adopts wholesale the appearance, lifestyle and attitudes of his new class. In the third book, the nephew of one of Montmorency's friends is upset by how blind his uncle and Montmorency are to an elderly servant's health. They take certain amenities for granted. Servants are just there, part of the background. One is civil, but one doesn't thank them for being servants.
Updale's understanding of the nineteenth century upperclass continues in other regards. A reformed prostitute (not really reformed; she just isn't practicing her trade anymore; it is never implied that she feels practically repentant) is also one of Montmorency's friends; without saying so outright, it is clear that one of three people (a lord, the doctor and Montmorency himself) could be the father of her son. There is no apology for this. Updale is not writing salacious literature; she is writing from within the thought processes and mores of the time. She doesn't dwell on these probabilities; nor does she point them out as bad behavior. She allows them to exist because that is the world she is writing about.
The result is a rather unsettling series. You like Montmorency but he is so thoroughly himself that he doesn't always behave heroically (or what we think of as heroic behavior; he has survived against great odds). Psychologically, everything he does (his self-protection, his vanity, his adopted attitudes) make sense, as do the psychological profiles of the other characters.
I find it bizarre that these books would be placed in the young teen section. Montmorency is not even a young character. Several books about thieves have appeared recently, all the protagonists being teens. Montmorency is possibly a teen at the beginning of book one, but he is well into his twenties by book two. In book three, he is about forty, and a few other characters (all in their late teens) have taken his place as the young protagonists.
This isn't to say I would be surprised if pre-teens/young teens adore the books. My surprise isn't at the readers but at this whole business of determining that books are for child readers or for young adult readers or for adult readers. What on earth constitutes the criteria? Is is subject matter? Montmorency books deal with--amongst other subjects--class systems, drugs, ambition, anarchists, prostitution, opera. Is it the style? Updale does use a straightforward style that is deceptively simplistic in its presentation. But then, so does D.H. Lawrence.
Personally, I think is comes down to explication. If Updale never "shows" characters having sex; if she never employs long, Freudian analyses; if she never blathers on (a la D.H. Lawrence) about nineteenth century politics or compares, heavy-handedly, nineteenth century politics to modern day politics...voila, her books must be for children. Which is, in my estimation, ridiculous. Doing such things doesn't automatically make a book "adult-like" any more than not doing those things makes a book "child-like." Yet somehow we have this idea (it started in the eighteenth century or so) that a lack of long-winded explanations constitutes non-profundity. Hence, fairytales and folktales become the property of children while Dickens edges his way into the category of adult literature.
It's the sort of thing that made Lewis and Tolkien grind their teeth. Folktales and fairytales, despite the lack of analytical explication, originally belonged to a community as a whole. You can't really say that they were originally considered adult tales (although they kind of were) because "childhood" as a concept didn't really exist until the eighteenth century. But eventually, the split was made. And I think, alongside Lewis and Tolkien, that it did more damage than good (although I can understand the publishing/commercial need for designations and genres; unfortunately, I think these designations go beyond that).
The determination that certain kinds of content/styles were "childish"--namely content and styles employing supposedly simplistic forms--meant that "mature" literature got linked with complicated, expositionary, profound-type (or obviously profound) passages, usually depressing or historical; these latter definitions eventually got linked with GREAT literature. And an entire generation of teenagers were doomed to read Hamlet instead of Much Ado About Nothing; The Pearl instead of Greek myths and The Scarlet Letter instead of Bruce Brooks' Midnight Hour Encore of Brock Cole's The Goats. Good writing, that is the kind of writing that adults (and English programs) study, became equated with Dickens and Faulkner and such.
Don't get me wrong, I like Faulkner, but good writing as non-expository has become harder to promote and harder to explain. Agatha Christie was an excellent craftswoman; Where the Wild Things Are by Sendak is a perfect read; E.Nesbit coined a conversational writing style that influenced generations of writers; Elizabeth Enright's books have flawless prose; The Queen of Attolia by Megan Whalen Turner is an amazing adventure/love story with entirely believable characters. None are books/writers studied in regular English programs. (You have to take Mystery Novels or Children's Lit.)
The love of good writing has been swallowed up by the search for MEANINGFUL applications; MEANINGFUL applications are easier to talk about than good writing; they are also easier to find in heavy-handed exposition. As a result, not only the good writing but the possibilities of meaning and thought in YA and children's literature has been overlooked.