PRESTON STURGES IS A BIOGRAPHER’S dream–a flamboyant, mercurial, ultimately self-defeating comic genius–and so it was baffling that the first two biographies written of him were shallow and inconsequential.

Now comes Diane Jacobs, a respected film scholar, journalist and teacher who has spent eight years putting together the puzzle of this extraordinary writer-director. Her “Christmas in July: The Life and Art of Preston Sturges” (University of California Press) may not be the last word on the subject, but it approaches him with a finely judged combination of empathy and critical perspective.

“An extreme embodiment of the American success dream,” as critic Manny Farber called him, Sturges was, naturally enough, an equally extreme embodiment of American failure. His life was as eccentric, improbable and exhilaratingly breakneck in its pacing as one of his own movies, which included such 1940s classics as “Sullivan’s Travels,””The Palm Beach Story” and “The Miracle of Morgan’s Creek.”

As Sturges wrote in 1958, the year before his death, “The funny thing about success is that either everyone wants you or nobody wants you and you yourself never know exactly why. There is also the element of luck. I had so very much of it for so many years that it is quite natural for the dice to roll differently for a while.”

STURGES’ LUCKY ROLL was so spectacular and yet so brief, and his fall so abrupt, that it seemed to have the tragi-farcical nature of his movie plots, with the kinds of irrational ups and downs that govern the lives of Joel McCrea’s movie director John L. Sullivan in “Sullivan’s Travels” or Eddie Bracken’s ultranerd Norval Jones in “Morgan’s Creek.”

Jacobs patiently unravels what she calls “such a confluence of things” that contributed to Sturges’ downfall–including the failure of his beloved restaurant (The Players) and resulting heavy indebtedness, his battles with Paramount executives and ill-fated desire to go independent, and his increasing tendencies to take refuge in outrage and drink.

It’s a cautionary tale about the pitfalls of Hollywood, egotism and the almost pardonable arrogance success can bring. The final chapters are full of grim details (Sturges was so broke in Paris he had to scrounge a meal at a party for Billy Wilder’s “Love in the Afternoon”), but Sturges always retained a cockeyed optimism and nobility.

“In a sense it was a tragedy, because he had such a terrible fall,” Jacobs observes, “but he never gave up. He thought things would get better.”

Jacobs began her research in 1983 and spent the first two summers reading Sturges’ letters and other papers at the UCLA Special Collections Library. “It was an incredibly amusing time,” she says. “Reading his letters was such a joy. I feel that’s how I came to know him.”

One of the triumphs of her research is that she tracked down, won the trust of and interviewed Sturges’ three surviving wives (Eleanor Hutton, Louise Tevis and Sandy Nagle), as well as his mistress Frances Ramsden (his first wife, Estelle Godfrey, died before Jacobs started the book). Sturges’ women helped Jacobs paint a rich, in-depth portrait of his personal life and his contradictory treatment of the opposite sex.

JACOBS INITIALLY ASSUMED from Sturges’ films and their strong female characters (such as Barbara Stanwyck’s footloose cardsharp in “The Lady Eve”) that the filmmaker shared her feminist views. Then the author began hearing about the time he gave Louise a black eye and the time he knocked Eleanor down a flight of stairs.

“How can I write this book about somebody who could do that?” Jacobs recalled asking herself. She went through a period of soul-searching before coming to the conclusion, partly from talking to the women in his life, that Sturges was a man of many parts who could not be pigeonholed or easily explained:

“I didn’t feel he was a ‘violent man.’ I felt he had a lot of violence in him , and a lot of passion, and he was very romantic. It must have been the most wonderful thing in the world to have Preston Sturges fall in love with you.”

Sturges was also a political conservative “very much out for himself” who wouldn’t join the writers or directors guilds in Hollywood and never quite realized what all the fuss was about during World War II, Jacobs reports. His jaundiced view of humanity, which he shared with H.L. Mencken, made his films ahead of their time in their irreverent humor but also may have pushed him down the path to creative sterility.

“I grew up in the 1960s and wanted to change the world,” says Jacobs, “and of course I hoped to find that he wanted to change the world.” But she also found Sturges “a real iconoclast” and realized that as a biographer, “You don’t change (the subject) into yourself or try to judge him. I wanted to get into his shoes. I hoped to show who he was without saying this is how you should be.”

WHILE JACOBS’ ANALYSIS of Sturges’ character is acute, what made him “the most spectacular manipulator of sheer humor since Mark Twain,” in Farber’s words , remains somewhat elusive in these pages. Writing about the roman-candle incandescence of his scripts and his zany characters (with outlandish names like Trudy Kockenlocker, Hopsy Pike and the Wienie King) perhaps requires a critical vocabulary that hasn’t yet been invented.

But in bringing the personality of one of America’s greatest filmmakers back to life, Jacobs has helped point the way toward deeper appreciation of his films. Her loving but clear-eyed portrait bears out the truth of what she writes in her introduction:

“In a discarded prologue for his film ‘The Great Moment,’ Sturges wrote: ‘Of all things in nature, great men alone reverse the laws of perspective and grow smaller as one approachesthem.’ This is not true in his own case. The closer I came to Sturges through both his life and his oeuvre, the larger, if more contradictory, he grew.”