Baby Snatcher : by Susan Terris (Farrar, Straus & Giroux: $10.95; 192 pp.)

Who is safe and who is dangerous? The distinction can be hard enough for adults to make when encountering a stranger, but for a 13-year-old girl hovering on the brink of adolescence, the question can be traumatic.

The uncertainty of human nature underlies Susan Terris’ “Baby Snatcher,” a novel ostensibly written for young adults that has enough probity and insight to interest older readers, too.

Laurel Tavrow is used to spending uneventful summers near a rustic Minnesota lake, but she’s surprised to find someone living in a formerly deserted and run-down cottage on a neighbor’s property.

Its inhabitants perplex her even more: Ivan, a hippie sculptor who survived into the 1980s, and his infant daughter Doe.

But, he rarely finishes any of his wood sculptures and is more likely to jump in a canoe and take off across the lake, leaving his strangely solemn daughter to her own devices.

With nothing better to do and intrigued by Ivan’s eccentricity, Laurel agrees to baby-sit with Doe, whose mother’s absence is easily, if not convincingly, explained by Ivan. Soon Laurel is taking more care of Ivan than his baby, cleaning the cabin, cooking his meals (with disastrous results) and slowly and surely falling in love, in the way only 13-year-old girls do.

What might have been a trite romance of a teen-ager spurned becomes much more in the capable hands of Terris, whose writing is as smooth as the sculptures Ivan lovingly sands for hours on end. Laurel thinks her growing affection for Ivan is mutual, a feeling confirmed by a late-night swim together in the lake.

Terris is adept at delineating the isolation most adolescents feel, accentuated in Laurel’s case by her feelings of inadequacy in a family of superachievers. Those feelings rush to the surface when Jessica, Laurel’s stunning 23-year-old sister, arrives at the lake and quickly captivates Ivan.

Laurel soon sees that her sculptor hero has clay feet. She is particularly stung by the abrupt withdrawal of the respect Ivan had offered, and its replacement by the condescension adults typically display toward teens.

Forced back to reality by her jealousy, Laurel also must confront her suspicions about Ivan and Doe, questions she had ignored in the passion of her crush on the older artist. Ivan’s defensiveness makes Laurel more curious about Doe’s mother and her supposed approval of Ivan’s custodianship.

Terris builds to the suspenseful conclusion of “Baby Snatcher” quickly and relentlessly. In the process, Laurel matures, learning more about love and responsibility than a routine summer at the lake might have indicated.