Although the book is listed as a fantasy, structurally, it's more of a children's clue solving mystery, where the kids move from clue to clue, deciphering their meanings, to an ultimate MacGuffin. The book could just as easily be about hidden atomic secrets and Russian spies. Although there is "magic", it is mostly hinted at and implied, even if implied strongly. By not means is it flashy or even a primary motivator of the plot. Perhaps the book better belongs in the paranormal genre?
As children's mysteries often have some urgency, the book also contains elements of a conspiracy theory thriller. The enemy is intelligent and active, seeking a long lost item of great power. If they get it, then the side of good suffers a tremendous loss. But the enemy isn't dumb, doing its best to out-connive and out-innovate our heroes. The opposition here is very active, adapting their strategies to the moment at hand.
The book works exactly like a film script from the 1960s, as stretches of instigating and parent dodging get interspersed with moments of excitement. The scenes are pretty much structured like film scenes and uses all the expected tropes of the children's mystery genre as seen on TV. As a film, it would be dirt cheap to produce, even using 1960's film technology. (Today, vast waves of special effects would be thrown into it, doing nothing for the film but making it more expensive.)
Overall, I found the book is very tight in its plotting, but a bit weak on engagement. The work was clearly written from an outline, which ensured that hit all the beats that it need to hit, but it doesn't engage emotionally in any other way than a thriller would.