WHAT IS A PARENT to do in that inevitable, challenging moment when a precocious 10-year-old takes off her Walkman headphones and asks, "Mommy, what's a fugue?" The Oxford University Press has provided a handy answer, and not a moment too soon. You simply take The Oxford First Companion to Music from its spot on a nearby shelf, hand it to her and say, "Here, Cosima, you can look it up in your own little music book."

A bit of caution is, of course, advisable. If Cosima or Wolfgang asks "What's a plectrum?" or "What's a nonet?" the answer will not be found at its alphabetical place in the five-page music dictionary that is included in this compendium of entry-level musicology. But the information is available where it belongs--"plectrum" in the one-page article on "plucked strings" and "nonet" in a little glossary that accompanies the one-page article on chamber music. The McLeishes, faced with the challenge of producing "the first comprehensive music information book to be designed for the under-thirteens," have considered their assignment carefully and tried (with considerable success) to put each nugget of information in its natural context.

They have produced a combination reference work and book for browsing, divided into seven topical sections such as "Music Round the World" and "Instruments and Orchestras" for quick access and/or prolonged reading. "Listening to Music" includes descriptions of some pieces attractive to children ("Peter and the Wolf," "The Sorcerer's Apprentice," etc.), while "Singing and Dancing" has lots of opera and ballet pictures as well as thorough diagrams of the vocal chords and related apparatus.

Pictures (many in color) are one of the book's strengths, including portraits of composers, pictures of instruments, many scenes of music-making and diagrams of "How a conductor beats time." The book is attractively laid out for young readers and well-geared to their levels of understanding. It does not answer all possible questions, of course, but if young Johann Sebastian starts asking "What's an appoggiatura?" that means it's time to trade up to the larger and much more inclusive Oxford Companion to Music.