By Ruth Rendell

Crown. 340 pp. $25

Getting under the skin of the insane -- including the criminally insane -- seems to come naturally to Ruth Rendell. Time and time again, with a cool air of control evidenced in her steady prose, Rendell has spelled out the bizarre thinking of people who are odd in the extreme. Often she seems to understand miscreants so well that she can make their logic sound sensible. When that happens, it's time to shudder.

Psychopaths are no strangers to Rendell's Inspector Wexford books, but in that series, which began with "From Doon With Death" in 1964, they are usually viewed from the sleuth's sane perspective. Reading about them can be uncomfortable enough, but when Rendell writes books out of the series (sometimes, but not always, under the pseudonym Barbara Vine), the perspective often switches to that of the insane -- with results that are truly frightening.

It must always be a tall order to write with level-headed command about someone who is out of control. But Rendell has built a career on her skill at doing this. Critics love to read her work with one eye on craft and the other on the enigma of the character who is at the heart of the psychological suspense. Unfortunately, reading "13 Steps Down" in that way led this reviewer to feel cross-eyed, for Rendell's narrative technique does not always seem up to the job.

In this tale of a stalker called Mix Cellini -- whose obsessive pursuit of a stunning model, Nerissa Nash, leads him to commit crimes far more heinous than stalking -- Rendell breaks a rule taught to every student of expository writing:

"Show, don't tell." Repeatedly, she summarizes a character's (especially Mix's) state of mind, rather than letting us divine it through his actions. "Mix intended to be famous," she writes. "All this introspection wearied him," she states. "I hate children, he thought," she informs us in the last chapter. The reader is left to wonder if Rendell has done this consciously to echo the literal-mindedness of the psychopath or unintentionally, due to some lapse of skill.

It looks like a case of questionable craftsmanship when Rendell tells rather than shows Nerissa's inner thoughts: "She was in love with Darel Jones, knowing this quite clearly because she had never been in love before." Here there is no question of portraying the psychopath's method of thinking.

Rendell does a better job of picturing Mix's psychopathic detachment, while directly telling us what he thinks: For example: "The weather was still what people called glorious. He would rather it had been cold and gray, for this warmth and sunshine brought the neighbors out into their gardens."

More than once she inspires profound pity -- a difficult emotion to stir in readers -- especially when writing of Mix's landlady, Gwendolen Chawcer. A Miss Havisham figure, this elderly woman has long held a hope that the once-young Dr. Reeves will return to sweep her off her feet. It seems the doctor took tea with her during visits to her ailing father when Gwendolen was young, too. She lives in a home and among furnishings that are decaying all around her. All that is missing is the moldering wedding cake.

Still, there's plenty moldering chez Chawcer, thanks to Mix. Rendell proves she does not flinch at depicting blood and gore -- and the odors that accompany them. She also shows herself a master of the emotionally complex moment. For instance, the comic and the pitiful make strange but giggle-inducing bedfellows when Gwendolen gets all atwitter at the arrival of a well-spoken Asian neighbor. It is almost enough to distract her from her obsession with Dr. Reeves.

Real pain, resulting from thwarted love, is at the heart of all the characters' fates. Many of them are enthralled with the unattainable, and Rendell's astute depiction of this human tendency makes up for what seem to be lapses in technique. It is also the reason readers will feel rewarded when they finish this flawed yet fascinating novel.