A rare first American edition of Robinson Crusoe is the sought-for object in this author's latest mystery for third and fourth graders. In the New York countryside, young Sten and Vicky are asked to mail the book to its owner, their friend and neighbor, Professor Barton who is in London. But the professor's letter with directions for getting into the secret room where he keeps a whole rare book collection is lost. Then a suspicious marauder turns up. Stan's and Vicky's detecting, with the professor's gardener and some visiting relatives, reveals the secret. The marauder turns out to be a disturbed ex-student of the professor's, presumably bent on theft. Prospects of a nice summer vacation with the professor, when he returns, leave a pleasant taste, but the loud; stage whispers in the narrative make this for youngsters who won't bite through thin threads too easily.