This one was published in 1967, technically later than the Weekly Reader Book Club books I usually collect as mementos of my late 50s - early 60s childhood, but a quick scan at the bookstore suggested it was in a similar spirit as the earlier ones. Also, it's got black & white ink drawings throughout and I love me some black & white line drawings. It's not at all bad, and it's set in an Adirondacks lakeside resort so that hits one of my bells right away. The protagonist is a 12-year-old boy, Mark, whose father died two years before the book opens, and his mom has married the owner of said resort and they've all moved there so his stepfather can better manage the business. Mark's favorite college student summer worker is Ben, who treats Mark the best of all the summer help. But Ben seems to keep getting in trouble with the adults who work there, and when an extremely valuable part of an elderly regular's stamp collection goes missing, Ben gets blamed - and fired. Mark and two daughters of the resort's engineer, whose family lives on site, stick by their friend and go sleuthing to try to find the real thief.
It's an enjoyable story with nice descriptions of the natural beauty of the setting, the lodge and the characters. There's a hair-raising near-drowning incident with Mark and his friend Kristy paddling for their lives in a dark cold lake when their boat inexplicably sinks. As far as "fair play" mystery standards, I think it probably passes, but it was meant more as a kids' adventure than a Golden Age mystery so that doesn't bother me.
It did strike me as a bit odd that in 1967, when anti-war riots and hippie rebellion was spreading fast across the US and were in the news daily, none of that appears here, even in passing. But it's a fantasy of sorts, and there's no law that says they have to address real-world issues.
I'd recommend this one to anybody who likes a lively kids' adventure with mystery, derring-do, loyalty and friendship foremost.