It's not exactly a children's book, but may be classified as a "juvenile" novel and is the young peoples' room in the berkeley public library. but I read it as a 5th grader, and it made a deep impression on me: _The Currents of Space_ by Isaac Asimov. A small subplot in it concerns setting of alarms by requesting a library book on a forbidden subject. Another book that has probably interested many young people in privacy tech is the Zim book on codes and secret writing. I read it in grade school, and I seem to recall reading that Diffe did too. Author: Asimov, Isaac, 1920- Title: The currents of space / Isaac Asimov. 1st Ballantine Books ed. New York : Ballantine Books, 1983. Description: 231 p. ; 18 cm. Series: A Galactic Empire novel. A Del Rey book. Ballantine ; 31195. Del Rey science fiction. Notes: Cover art by Darrell K. Sweet. Other entries: Sweet, Darrell. Author: Zim, Herbert Spencer, 1909- Title: Codes and secret writing. New York, William Morrow, 1948. Description: [i-vi] 1-154p. illus., diagrs. 20.3 cm. Notes: "Books worth knowing": p. 144-145. Bound in orange cloth; stamped in brown. Dust jacket. Library of the American Cryptogram Association (George C. Lamb Collection). Subjects: Cryptography. Call numbers: UCB Ed/Psych Z104 .Z5 Children's Lit. Coll.