Writing as his wounds heal--a seductive, unexplained come-on--John Allen chronicles his recent adventure, a genuinely intriguing series of events which will entertain sophisticated readers, able to handle a large cast and the 1865 milieu, and will transfix kids with a liking for codes. A New York City boy with an active imagination and great admiration for a Union army captain, John goes upstate to visit country relatives, consoling himself with fantasies of uncovering robbers, Rebel-sympathizers, and other undesirables. ""'If you want to remember a man,'"" he's counseled, ""'there's four things he can't change much--ears, hands, eyes, voice,"" and this offhand advice, along with some fairly complicated deciphering techniques, is the basis for highbrow detection by John and equally energetic cousin Sam. Unpredictable plot turns and mystifying details maintain the suspense, and crisply etched personalities increase the page-turning potential. We won't tell all but the Secret Island, where the twice-stolen loot is buried, is underwater, and the clues hinge on Biblical references. Password: rewarding. Don't keep it secret.