IF YOU secretly snicker when your 12-year-old tells tales from the Totally Gross Jokes books, or if you have a young teen who's presently plowing through what we'll call the Grungy Stage, try Jerry Spinelli. As a father to six, Spinelli is Grade A Gross with seventh-grade sleaze, a master of those embarrassing, gloppy, painful and suddenly wonderful things that happen on the razor's edge between childhood and full-fledged adolescence.

In his second young-adult novel, Who Put That Hair in My Toothbrush?, he's toned down some of the more ff-putting sleaze (pubic hair is out, green toenail polish is in) and tuned into his characters to produce a book most likely destined to become a junior high fave before the year is out.

In it Spinelli demonstrates what is probably a well-filled ear for sibling rivalry -- the kind that's escalated beyond the negotiating stage into a truly scary arms race. The two sides -- seventh-grader Megin (called "Megamouth" by her brother) and ninth-grader Greg (she calls him "El Grosso") -- alternate chapters while altercating over everything from their little brother to her hockey stick.

Between the black outline of their arguments, Spinelli colors in their daily lives. Megin turns out to be a jock with the sensitivity to adopt a tough old lady at the local nursing home as her grandmother and the kind of raw craziness it takes to impress the class' most popular girl (see above for toenail color).

Her brother is busy being cool (a fulltime job for ninth graders) and cooking up ways to be with his dream girl. Alas, she really is a dream, not his girl -- a painful fact that forces him to grow up a bit. When a real, flesh-and-blood substitute turns up under his very nose, he eventually learns to appreciate her.

The same cannot be said for his feelings toward his sister. At the end of the book, an accident on the ice, in which brother and sister rescue each other from death by freezing, shows the strength of their blood bond. But it's clear that, even then, their rivalry will continue -- only maybe this time with a little negotiation.

Relations aren't that great between Alexandra Dimitrios and her sister in award-winning-writer Scott O'Dell's latest novel, either. For one thing, they're both in love with the same man. The difference is that Alexandra's sister (the pretty one) is engaged to marry him, whereas 16-year-old Alexandra (the son her father never had) is left to adore from afar.

Set in the Greek sponge-fishing community of Anclote Key off the tip of Florida, Alexandra deals with issues central to our time. There's feminism: The heroine defies her mother and community by becoming the first female there to dive for the sponges (athough her grandfather and mentor assures her that Greek women did it all the time in the old country).

Then there's drugs: Alexandra gets suspicious when whatever they bring to the auctioneer is scooped up at inflated prices by a mysterious Mr. Kanarsis. What's more, her sister's boyfriend suddenly becomes very friendly every time they're out making these catches.

WHEN THE big attraction turns out to be cocaine stashed among their sponges, Alexandra has to work out where her loyaltes lie -- with her future brother-in-law, with her grandfather (who would suffer from the scandal), or with the law. She makes a crucial decision; and then, as she takes the first, dangerous step, the book stops -- an ending that either demonstrates amazing respect for the reader's imagination or signals a sequel.

All in all, it's a tightly witten book on deep subjects from an author who's demonstrated in most of his 18 other novels, including Newbery-award-winning Island of the Blue Dolphins, that he knows how to handle the depths. Marilyn Sachs, in her latest book, is into the depths as well. The Fat Girl is a book about manipulation -- parents manipulating kids, kids manipulating each other, the strong manipulating the weak and everyone losing as a result.

A chilling book in many ways, The Fat Girl starts with 17- year-old Jeff, son of a manipulative martyr-mother, brother to a girl nearly smothered in the nest by guilt and anger and eventual boyfriend to a fat girl.

With the fat girl, Jeff -- who has learned to phrase every sentence and give every appearance to his mother's liking -- unleashes his own manipulative tendencies. He puts the girl on a diet, pulls her into stores, tells her what to wear (including makeup) and directs even her hobbies.

Eventually, like his sister, the girl rebels -- telling him off, and taking control of her own life. But unlike his mother (who ttempts suicide when his sister leaves home), he learns from the experience, and you sense that his manipulative days are over.

It's a compelling novel from an established writer who only rarely writes for this age group. In this book, she's Grade A great.