BOOK REVIEW / NOVEL : Sexual Obsession and Murder Amid the World of Art : FINE LINES: A Novel <i> by Simon Beckett</i> ; Simon & Schuster $22, 283 pages

Agatha Christie’s play “The Mousetrap” has been a fixture in London’s West End for decades, diverting audiences with eccentric characters and a surprise ending but without straining anyone’s imagination or sensibility.

“Fine Lines,” a first novel by British journalist Simon Beckett, isn’t anything like “The Mousetrap” in terms of plot, but it, too, is an undemanding production written so professionally one forgives its broad-brush predictability.

The resemblance continues in the fact that the novel, despite being a will-he-get-away-with-it rather than a whodunit, seems to beg for popular stage adaptation; its parties, murder, arguments, sex scenes and general voyeurism would fit snugly in the protagonist’s London art gallery . . . thus lending the work a patina of respectability it wouldn’t otherwise deserve.

“Fine Lines” needs respectability, too, for it deals with a theme much favored by commercial novelists these days, one that might well make Christie blanch--sexual obsession.

It’s a crowd-pleasing subject, and difficult to bungle as long as a reasonably talented writer places it in a fitting milieu. Josephine Hart set “Damage” in high-level politics; Beckett sets “Fine Lines” in the art world, and it’s a good choice, given the art culture’s unavoidable preoccupation with representation and falsification, sensuality and taste.

Donald Ramsey, the narrator of this novel, is a discerning, middle-aged art dealer who has made his reputation, and a considerable fortune, selling works of unquestioned merit.

He’s a man of little interest or consequence aside from one quirk: Ramsey collects antique and old-fashioned erotica, a harmless hobby that compensates for his nonexistent, apparently unwanted, sex life.

Unwanted, at least, until the day he returns to the gallery after hours and discovers Anna, his young assistant, quite naked; in the midst of changing for a dinner date with her boyfriend, she is unaware of her employer’s presence. Ramsey is at that moment transformed, feeling intense sexual urges for the first time since adolescence.

The rest of “Fine Lines” is essentially a low-rent reversal of “Cyrano de Bergerac,” with Ramsey hiring a gorgeous male model, Zeppo, to seduce Anna in his stead.

Zeppo and Ramsey have terrible arguments, and end up committing murder in the course of this criminal endeavor. Their partnership turns out to be the best part of the book; Ramsey is an aesthete and gentleman, Zeppo a hedonist and cynic, and the two mix like oil and water.

It’s easy to see why a rake like Zeppo would derive such sadistic joy from taunting Ramsey about his sexual innocence, and why Ramsey, likewise, might jeer at Zeppo for his dimwitted cockiness. Zeppo, in addition, has a way of making Ramsey act coldbloodedly; it’s as if Ramsey’s brain--only superficially engaged with the world until he sees Anna naked--is supercharged by access to Zeppo’s brawn.

*

“Fine Lines” has one inescapable problem; it’s a one-trick pony. Ramsey’s obsession with Anna is plausible, given his thoroughgoing repression, but his dogged attempts to satisfy that fixation unfold with so few surprises the story becomes monochromatic. That’s not an entirely bad thing--think of film noir--but doesn’t suit a novel set in the art world, which could provide all sorts of engaging novelistic diversions.

The subplots Beckett does utilize--Ramsey’s ill-starred run-ins with a widowed decorator, his encounters with Anna’s parents--are well-executed but extraneous, little more than padding.

Perhaps the biggest surprise in “Fine Lines”--and it’s a pleasant one--is Beckett’s omission of the just-desserts ending one expects from a first-person confessional novel. Ramsey (caution: plot giveaway dead ahead) does in fact get away with murder, resuming his old art-dealing ways an unchanged man.

Beckett obviously intends to produce much titillation in the course of “Fine Lines,” and does so, but its most noteworthy aspect is Ramsey’s lack of guilt, his detached acceptance of the obsession that drives him to singularly uncharacteristic acts.

His compulsion satisfied, Anna becomes just another hireling, her mouth too big, her lips too rubbery, her thighs too heavy . . . and her employment at the gallery, of course, soon to end.