LIBBY SHADOWS A LADY

What a strange, strange thing,"" marvels Libby's Aunt Paula toward the end. ""A kidnapping and a bombing. On the surface, no connection."" Staying with busy Aunt Paula, also a New Yorker, while her parents spend the Easter vacation in Europe, Libby occupies herself by trailing a young foreign woman she has overheard talking on a pay phone about bombing the Federal Reserve bank. It turns out that Ms. Pietro isn't planning to bomb the bank but her remark has unwittingly encouraged her Italian language students to bomb Columbia; more excitingly, in following her -- from the UN building to Morningside Heights to Carl Schurz park -- Libby is able to prevent the abduction of Ms. Pietro's baby, in the process helping the police both round up an active kidnapping ring and close in on the otherwise unrelated bombing conspiracy. Girls will hurry along with Libby on her peripatetic search for dues, reading this at a level where the stock characters, soap opera ending and numerous handy coincidences are only minor irritants.