At the risk of appearing to stoke the thoroughly-raked embers of the “Is Die Hard (1988) a Christmas movie?” conversation — it’s not, by the way — how much Christmas should appear in your mystery in order for it to be considered a Christmas Mystery?

If the seventh adventure to feature Ken Holt and Sandy Allen starts three days before Christmas and continues until three days after it, taking places at least partially on Christmas Day itself…that’s enough, right? Well, no, because there’s nothing really inherently Christmas about any of the mystery — the same reason Die Hard doesn’t qualify, as it could take place at almost any time and the exact same plot play out, where the Tim Allen-starring The Santa Clause (1995) does because the entire plot revolves around Christmas (see also: Jingle All the Way (1996), Reindeer Games (2000), and Nativity 3: Dude, Where’s My Donkey? (2014) — seriously, with that company, who wants to be a Christmas movie?).

Anyhoo. The Mystery of the Iron Box (1952) by Beryl and Sam Epstein under their Bruce Campbell nom de plume may happen to be set around Christmas-time — pure coincidence, by the way, that I’m reviewing it around Christmas-time — but it largely follows the same beats as other entries in the series: a mysterious event, some investigation, the realisation of a bigger scheme, and then a usually-protracted showdown or confrontation of sorts in which peril is encountered and must be overcome. If that sounds dismissive, know that I’ve now read five of these and have no intention of stopping any time soon; they may not possess the investigative rigour I’d prefer, but the boys are good fun and the Epsteins largely guide them through their adventures in an entertaining way.

Christmas brings Ken’s ace reporter father Richard Holt back to the United States briefly, and following the Allen clan’s adoption of Ken it’s there that everyone heads for the holiday. And when Ken alone becomes convinced that one of the presents his father has brought back with him is a target for thieves — “Every time they see a doughnut they begin to worry about who stole the middle out of it” Sandy’s older brother Bert explains to Richard, dismissing the signs Ken claims to have seen — he and Sandy will encounter a growing raft of minor offences before some good interpretation of unusual behaviour puts them on the right track.

For three-quarters of its length, The Mystery of the Iron Box mixes well the requirements of an adventure story in its swift, varied, related, mysterious events and a fairly-clued detective story with some good hiding in plain sight (including a great piece of highly unlikely behaviour which it would be entirely possible to sail right past) and some inventive ways to stoke the mystery that feel just organic enough to reward their creation (the photograph of the box — yes, it’s unlikely as all hell…but, c’mon, it’s also rather gorgeously wrought and deserves credit for that). There’s some good learning to do, too, such as the logic applied to an intruder’s movements in a house or an interesting line on what the eponymous mystery could revolve around since the box is only a cheap reproduction. As Ken points out, “The only reason [Dad] thinks it isn’t valuable is because he apparently didn’t pay very much for it.” — but, as we’re going to learn, there’s value and there’s value.

Some nice details sneak in, including the apparently casual way that wildly inflammable materials were simply made available to the public and everyone just knew about it — man, the 1950s were wild…but, then, a letter of introduction from a doctor could get you pure arsenic if you wanted it, so I suppose starting a few fires is nothing. The sense of New York city and the various means of traversing it, too, are nicely and quietly conveyed through taxi rides, subway journeys, and more — sure, Ken must have some sort of bottomless Taxi Fund to afford so many cabs in the middle of the city over Christmas, but verisimilitude need stretch only so far, I suppose.

Their investigation progresses in a way that is pleasingly constructed, too, with good reasoning but not everything going Ken and Sandy’s way: the inconveniences of having to shadow a suspect through snow-coated city streets, or the necessity of taking a leap — a correct leap, of course, otherwise it’s a shorter book — when all seems lost. This aspect of all the books has been consistently strong, and it’s always fun to watch the Epsteins apply good reasoning in realistic conditions to enable their plots to continue. The setups may not always compel, and the conclusions may not always convince (or, indeed, interest, as in The Clue of the Coiled Cobra (1951)), but the middle section of any Ken Holt book is, I’d wager, going to be as strong as anything you’ll find in the juvenile mystery wheelhouse.

The final quarter — in which, brace for a surprise, Ken and Sandy are captured and must evade their captors while in an increasingly-perilous situation — is then like something out of Intensity (1995) by Dean Koontz: just wall-to-wall with escalating stakes, diminishing time, and every single decision being discussed and analysed. Koontz wrote this sort of thing magnificently, and I’m going to say he learned it from the Epsteins — it’s a change of pace, but never less than utterly gripping. One downside, though: it turns out I really don’t like reading about people having their wrists bound so tight that it cuts off the circulation to their hands, and the three chapters spent exploring various options to get out of said bonds have left me with a tingly feeling in my web-spaces that I fear I may never shake.

While The Mystery of the Iron Box turns out to hinge on a disappointingly mediocre MacGuffin, the manner of its conduct and the speed of its progression, not to mention the effective shift in gears when the extended finale kicks in, make it another recommended entry in this series. I already know that the succeeding title to this, The Clue of the Phantom Car (1953) is a stronger entry again, so have the authors now reached their plateau and we get five or six classics from here? The next one I own is The Mystery of the Grinning Tiger (1956), leaving a two-book gap in the series down which any amount of quality could fall; hopefully, however, there are good things in Ken and Sandy’s future.

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