I'm going to review both Truly Devious and The Vanishing Stair as if they were one book --- because they essentially are one book split into two (well, three, really) parts.
This is a plot-driven, multi-layered, partly-historical YA mystery.
Stevie, a mystery lover, escapes her parents' enthrallment of a right-wing politician (who appears to be a combination of Sarumanian traits of several GOP politicians woven much like the wizard's robe of white-yet-not-white fabric) to a rather more realistic version of Hogwarts, a rich man's private school in Vermont, built in the 1930s. Stevie wants to solve the mystery of the kidnapping and murder of the founder's wife, the kidnapping and possible murder of his daughter, and the murder of a student who accidentally saw the kidnapper(s). However, very soon after she arrives, a modern student dies in what may be a murder or a very unlikely accident, and soon the mysteries begin to twist together. Another student, one who may have had motive and opportunity to murder the dead boy, goes missing. Stevie's parents want her home. The absolute jerk of a boy her hormones want continually uses her. And the slimy politician gets way more involved than is necessary.
The plots intertwine and surprise. It is a cracking good tale.
But book one end with NO ANSWERS, only "to be continued," and book two ends with only the identity of the first criminal revealed, but no real answers. The reader is left wondering many things.
(And since book three doesn't even have a title as of the writing of this review, it will be a good year before I can learn the answers!)
The setting is good and quite clever. It's very much a 1930s Hogwarts, only with multiple creaky old buildings connected by winding tunnels instead of a castle and with mechanical engineering instead of magic. Yet it's a great little world, very snug.
Johnson does go on way too often about "the altitude," which is supposedly 4500 feet -- a height that does not even qualify as "foothills," let alone "mountains," in the western US, but it's not too bad.
I do feel like Johnson is trying too hard with the characters; they feel like stock characters with every latest trend thrown in. Stevie, the protagonist, has a mental health issue: anxiety, but it feels tacked on, as it has not been crucial to the plot, and ordinary teen worries would suit just fine. Instead of the traditional gay best friend, Stevie gets a lesbian best friend (who may be the most well-developed character in the book), but the lesbian best friend is in a relationship with a gender non-binary person who uses plural pronouns. (This is SO FREAKIN' CONFUSING. Johnson could've gone with one of the new, singular non-binary pronouns, like "xhe," or just had the romance a lesbian one, as, again, the non-binary person has no real need to be so in the plot and appears to be that way just because it's trendy to write about non-binary and trans folks right now.) Then, the "nice guy" is a really bad stereotype of a novel writer who is almost a satire of himself, and the "bad boy" is not at all likeable, yet Stevie makes out with him whenever she can. The youtube star is a self-centered manipulator, and the artists are all hippies. Meh.
No, the characters are not as multi-layered as the plot.
My advice? This series is a must-read for mystery lovers; however, I recommend waiting until the full series has been published in order to avoid the agony of the cliff hanger.