As ‘The Family’ Turns : THE LAST DON.<i> By Mario Puzo (Random House: $25.95, 496 pp.)</i>

In 1965, broke, bored and underappreciated at age 45, Mario Puzo deliberately set out to write a piece of commercial fiction. He succeeded, big-time.

“The Godfather” sold 13 million copies, became the engine for three films and set Puzo up for life with handsomely compensated careers as a screenwriter and novelist. Now, at age 75, he returns with another skillfully crafted piece of commercial fiction, “The Last Don.”

It gives us Hollywood, Las Vegas and the mob in one sweet dish. Sex, murder, corruption, betrayal and redemption; beautiful women, vicious gangsters, sleazy producers, crooked cops and drunken pols--it’s all here and no doubt coming to a screen near you someday, because Puzo crafts a decent story while touching all the right commercial bases.

“The Last Don” is a nearly perfect summer book--bawdy, funny, easy to handle. It lacks the novelty of “The Godfather” but, like a good soap opera, its characters become addictive. You will root for two of them. Naturally, both are impossibly beautiful and smart, and their love helps them overcome the obstacles in their paths.

The book opens on familiar terrain, and for a little while you might think you’ve been there, read that: Don Clericuzio is an old man, wise and powerful, the wealthy leader of an unusual “family.” He’s noble but trapped in an ignoble world, a world where men who cross “the family line” are “dispatched.” He has three sons and a daughter, and he yearns to change his world so they and their children can make money legally and enter legitimate society.

One learns this background at a joyous family occasion--a christening as opposed to a wedding--but this echo of the “The Godfather” soon gives way to the story’s other twin peaks, Hollywood and Vegas. And it is here that one realizes Puzo must have been taking copious mental notes while learning about the movie business and pursuing a favorite pastime, gambling.

The Hollywood characters are particularly delicious. There’s Bantz, the ruthless studio executive who refuses to give “even the standard lip service to writers.” His job is to take over after directors finish their “artistic cuts” and make films acceptable to audiences.

There’s Skippy Deere, a producer and “cheerful ardent hypocrite” who cheats a friend out of big money, then says to her, in an echo of the “it’s-just-business” murders in “The Godfather”: “This had nothing to do with our personal relationship, this is between our lawyers.” Deere’s job is to take care of problems, like inserting a “moral turpitude” clause into the contracts of two starlets to prevent them from talking about an actor’s death.

Then there’s Ernest Vail, a “National Treasure” as a writer, but a babe in the woods when he goes Hollywood. His problem, according to Puzo, is that he “has no hidden agendas” and falls for the oldest gimmick in the world, taking points on net instead of gross. He mourns the backseat that books have taken to movies because movies give us “Sly Stallone as Achilles in the Iliad.” Vail is so put out that he threatens to kill himself if the studio doesn’t treat him right.

There are many more characters, including a hard-nosed entertainment lawyer who has her screenwriter friend write dialogue for her clients’ appearances on the stand. But most important, there’s Athena Aquitane, the book’s female lead, a “Bankable Star” with a tragic secret who’s being stalked by her crazed ex-husband.

The stalker poses such a problem, Athena quits a $100-million picture halfway through, inducing panic in Bantz, Deere and others. They suspect she’s not showing on the set because she and her agent are shaking the studio down for money. But Athena is not only beautiful and smart, she’s honorable. She’s genuinely afraid and doesn’t need to be a star to be happy.

Cut to Vegas, where Puzo gives us the male lead: Cross De Lena, the don’s grand-nephew, whose father is the Clericuzio family’s “No. 1 Hammer,” meaning killer. Don Clericuzio regards Cross like a grandson, and with his don’s help, Cross is running one of the town’s great casinos by the time he’s 25.

His age guarantees that multiple hot young actors will lust for the part, and why not? Not only is Cross beautiful and smart, he’s brave and tender. He has one big flaw, however: He kills. Midway through the book, Puzo tries to overcome this defect by having Cross express a sudden desire to leave the don’s violent world. It’s the book’s only serious misstep. Cross’ change of heart comes out of the blue, completely unmotivated. Only a few scenes--oops, pages--before, Cross has been anointed the family’s No. 2 hammer. The man he killed had murdered the daughter of a politician important to the family interests, but murder is still murder.

By now, you might have guessed what brings Athena and Cross together. Athena has that problem, the crazed ex-husband, and Cross takes care of problems when he’s not helping the don implement a grand scheme to make gambling legal nationwide so the family can monopolize it and go legitimate. Cross also wants to get into the movie business. He’s intrigued because his sister, who has turned her back on their hammering father, has become a successful screenwriter--so successful she can get points on gross.

As with Athena, there’s a tragic secret in Cross’ past, too, and it’s essential to understanding why he and the don’s grandson, the short and ugly Dante Clericuzio, become enemies as the story unfolds. Yet unlike Athena, who knows her secret, Cross doesn’t--not until the very end when he must risk the don’s wrath and avenge his father’s murder.

In revealing that mystery, Puzo gives us the resolution of all resolutions. First, in a St. Valentine’s Day-type massacre that would be the envy of any action director; then, a double murder in Las Vegas employing a practice the family calls “Communion”--making a body disappear. (Murders where bodies are found are called “Confirmations.”)

There’s a lot of violence in “The Last Don,” certainly enough to sustain a miniseries. Sex, too. Starlets sleep with moguls. Actresses are raped and seduced. Another “bankable star” collapses during a threesome. A lesbian director goes after her female stars. Athena and Cross make passionate love.

It’s hard to find much fault with such a Bankable Book. It is what it is--escapist fun in the hands of an adroit storyteller using what’s worked for him in the past. On occasion, Puzo seems to tire, and lame dialogue clunkily advances the plot. I began to scream at his repeated use of the word “huge” to describe chairs, tables, buffets, mansions, cabanas and Havanas.

I was amazed when Puzo, of all people, referred to a prosecutor as a “United States district attorney,” when such do not exist. Or when he made a gangster unafraid of a legal case because New York had no death penalty--even though the case in question was federal, not state.

Sorry to be so picky, Mario. But you didn’t leave much.