LOS ANGELES -- James Curtis, author of the new 1,056-page "Spencer Tracy: A Biography," says it could have been actress Loretta Young who became the love of Tracy's life, not Katharine Hepburn, his partner for 26 years and with whom he made nine films.

Tracy and Young, both Catholics, began a romance while working on 1933's Depression-era drama "A Man's Castle." Tracy was married to Louise Treadwell.

"I think they (he and Young) were drawn together because of the shared bond of faith," said Curtis, who spent six years on the book, drawing from Tracy's papers and obtaining the cooperation of the actor's daughter, Susie Tracy.

But it was their faith that caused the relationship to end. Tracy wouldn't divorce and Young would not have married a divorced man.

Tracy met Hepburn on set of 1942's "Woman of the Year."

"Hepburn ... fell madly in love with him, though she was never sure that he felt the same way about her," said Curtis. "She was absolutely devoted to him. I think she's a ... person who was, in some ways, completed by Tracy. He was not reluctant to tell her to shut up."

He won two Oscars (1937's "Captains Courageous," 1938's "Boys Town"), and moved effortlessly between comedies and dramas. But Tracy was beset with demons, including drinking, guilt over his affairs and that his son, John, was born deaf.

Susie Tracy is thrilled the book offers insights into her mother, who stayed married to Tracy more than 40 years and founded the John Tracy Clinic for the deaf. "He didn't meet Miss Hepburn until 1942," she said. "There were many years before that where a lot went on that people should know about."

Her mother, an actress, "could do almost anything," she said. Her dad was much like the charming one in "Father of the Bride." "He was funny. ... He told a joke wonderfully."

It was director Stanley Kramer who gave Tracy a splashy final act, casting him in the 1960s in "Inherit the Wind," "Judgment at Nuremberg," "It's a Mad, Mad, Mad, Mad World" (after which he had a heart attack) and "Guess Who's Coming to Dinner." Tracy, 67, died two weeks after its completion.

Hepburn's niece, Katharine Houghton, who costars in "Dinner," recalls that her aunt was a "basket case" during the production because of Tracy's failing health. "I think they both wanted to do the movie," she said. "They thought it was an important film, and they loved Stanley Kramer. I think they felt that, 'OK. It's better than just sitting around waiting to die. Let's try it.' "