Set on the Canadian side of the estuary between Maine and New Brunswick, this case of a nouveau riche neighbor whose mysterious visitors turn out to be Communist-linked may not be entirely original but it is intriguing and enriched by two discriminating brothers. Keith (15) and exasperatingly-honest Nelson (13), working as summer boat-boys-plus for Frank Becker, known the former POW (in Russia) has shady dealings in the wildlife preserve. Even though Dad and the local top dog warn them against snooping, they snoop, sometimes with Becker's wary stepdaughter Kim. Their prying is clever but it's their response that matters--they have to betray their employer who has been open and kind and who really, sadly, believes this last job will get him off the hook. Becker dies because, to avoid being taken, they ram the boat on Pirate Rock; that causes psychic abrasions for months until Kim (""the only human female"") zeroes in with a letter. Word-wise but not slick, and despite the familiar apparatus (planted mikes, underground tunnel, jarring righthand man) and young-looking illustrations, acute and credible.