Off to the Races With Fulvio Gasparini : WHEN THE FAT MAN SINGS <i> by William Murray (Bantam Books: $14.95; 216 pp.) </i>

You gotta love a gambler like Fulvio Gasparini--bets a stack of $100 chips on four-the-hard-way at the crap table; bets both the “odd” and the “even” at the roulette wheel. Bets $1,000 to win at the track on a horse standing backward in the starting gate.

Not to worry. Gasparini’s forte isn’t gambling, anyway. The acclaimed operatic tenor’s forte is fortissimo, a voice that drops song birds in mid-flight. It’s nature’s cruel trick however, that, with a compulsive gambler like Fulvio, just a tiny fraction of the talent that went into the booming voice didn’t seep into his dice-dumb fingers.

And so, with a couple of hours to kill between his magic show at the Three Kings in downtown Las Vegas, whom does Shifty Lou Anderson run into at the crap table at the Xanadu but Fulvio, holding the casino crowd entranced with a display of the worst dice throwing seen this side of an Army boot camp. With a sympathetic Shifty stepping in as his mentor, however, Fulvio’s luck turns dramatically. And born at the same time is a delightful and zany friendship between the rotund, irrepressibly likable tenor and Shifty, a “close-up” magician (coins and cards) playing the Las Vegas/cruise ship/Magic Castle circuit--as long as his agent’s booking dates don’t cut into the seven-week racing meet at Del Mar.

Enraptured by Shifty’s luck and skill, Fulvio insists that the magician joins his jet-set entourage as his personal, well-paid handicapper-in-residence. And, in New York, between sold-out performances at the Met, student and teacher work on the fine art of surviving at the track. But, in “When the Fat Man Sings,” novelist William Murray is too crafty a manipulator of magic, gambling and murder not to start casting shadows early in the game.

There are complications, for instance, in Shifty’s personal life--like leaving Las Vegas just as his extra-marital affair with a shapely Las Vegas illusionist/animal trainer is beginning to soar. So, the illusionist delights in making love with her 300-pound black panther in attendance. Everyone has her little quirks.

But there are other, more immediate and more sinister, things beginning to happen. Who would want to try to bash in poor old Shifty’s head even before he and Fulvio leave for the East Coast? And then again after they take up residence in New York? Other complications: Fulvio’s horse trainer in New York and his taste for the high-priced, slow-paced horses that he is buying in Fulvio’s name. And the maestro’s sultry, overly protective manager who--even without a 300-pound black panther in tow--takes the same libidinous interest in Shifty that the illusionist did.

And who killed, and for what reason, the aging vocal coach whose fortunes were entwined with Fulvio’s back in Naples many years before? Crime novelist Murray is a staff writer for The New Yorker and is the author of that magazine’s “Letters From Italy” feature as well as 10 novels, two previous ones also featuring the affable, horse-knowledgeable Shifty Anderson--”Tip on a Dead Crab” and “The Hard Knocker’s Luck.”

“When the Fat Man Sings” is more than a skilled exercise in three-dimensional characterization and true-to-the-ear dialogue. This is also an intricately plotted tale of people and things not being at all what they seem to be on the surface--of deviousness, murder and old vendettas bubbling to the surface.

In Shifty and Fulvio, Murray has created two likable rogues who play off each other like a pair of well-trained gymnasts--two radically different talents from two radically different cultures and bound together in a pact of mutual admiration and affection. But it is also the loving attention paid to the minor characters who flit in and out of the action that give “When the Fat Man Sings” its distinctively Runyonesque flavor: the hangers-on at Aqueduct talking out of the corners of their mouths, Shifty’s cynical agent, Happy Hal Mancuso, and Hugo Mandelbaum, the aging, rhinestone-studded, Las Vegas lounge comic with his pink shirt slashed open to the waist and his taste in humor cut even lower. “When the Fat Man Sings” is great fun and a genuine puzzler. And novelist Murray, when he’s not in Italy for The New Yorker, is in residence--where else?--at Del Mar.