When the actresses attending the 79th Academy Awards walk down the red carpet this Sunday, many will be interviewed about their clothes, only to toss off nonchalant lines like, “I just picked it out this morning,” or “Oh, I just thought it was pretty.”

They might be telling the truth, but it won’t be the whole truth. As Bronwyn Cosgrove’s exhaustively researched new book, “Made for Each Other: Fashion and the Academy Awards” (Bloomsbury) makes clear, choosing a dress for the Oscars is now a process that involves a committee. Dozens of designers make individual pitches. Personal stylists sort through the offers. The actresses reject or accept the results, often asking for impossible changes at the 11th hour or switching their allegiances on a whim. There’s nothing nonchalant about it.

Perhaps this is why most modern starlets indeed look like a committee has dressed them. Mainstream fashion commentators are always ooh-ing and aah-ing over “Old Hollywood glamour” on the red carpet, but these days that usually means a safe dress, elegant but not too sexy, and rarely surprising in the way runway fashion can be at its best (and worst).

As “Made for Each Other,” which is organized chronologically, explains, “Old Hollywood” wasn’t always so glamorous. Janet Gaynor wore a knee-length knitted dress with a Peter-Pan collar when she won the first award for Best Actress in 1929. Bette Davis wore a dowdy navy and white two-piece ensemble when she won in 1936. (According to Ms. Cosgrove, she was sending the message to studio chief Jack Warner that she was unhappy with having to act in the maudlin romance “Housewife.”) When Ingrid Bergman took the prize in 1944, she wore a dirndl skirt and peasant blouse. Joanne Woodward sewed her own dress in 1955. And Luise Rainer wore her nightgown when she won in 1937, after finding out only on the morning of the ceremony that she had even been nominated.

Of course, there was also glamour of the highest sort: Marlene Dietrich in black Christian Dior, Audrey Hepburn in white Givenchy. And during the early years of the awards, many actresses wore beautiful gowns designed by studio costume designers, such as Paramount’s Trevor Banton and Edith Head.

Nowadays, the studio system is dead, and the free market reigns. Ms. Cosgrove reports that Nicole Kidman earned $2 million to wear John Galliano couture throughout 1997. That included her appearance at the Oscars in a chartreuse satin gown from Mr. Galliano’s first collection for Christian Dior.

That was the same year Courtney Love impressed observers with her appearance on the red carpet. “Made for Each Other” includes more than 10 full pages on how exactly Ms. Love’s Versace dress was chosen. Her stylist, Wendy Schecter, calls the process a “game of psychological espionage,” but alas, it’s never nearly that interesting.

“Schecter dutifully visited Valentino’s Madison Avenue showroom, selecting six dresses, but instead of sending them to Paris, Valentino’s press office shipped the gowns to Berlin,” Ms. Cosgrove reports breathlessly. Sure, this minor disaster eventually affected Ms. Love’s eventual choice of Versace, but in the end, it wasn’t the white satin dress that was so surprising that year: It was Ms. Love’s clean hair, neat makeup, and polished fingernails, a trifecta we haven’t often seen since. If it were someone with a different track record, the gown would have been forgettable.

Ms. Cosgrove also includes laborious details on Renee Zellweger’s choice of a vintage Jean Dessès gown for the 2001 Oscars. The book ends abruptly that year, with Julia Roberts wearing Valentino. It might have been compelling to read what Ms. Cosgrove had to say about Jennifer Lopez in poofy pink Versace, Halle Berry in embroidered Elie Saab, or Reese Witherspoon in the vintage Dior she wore to the awards last year.

In the end, though, what could she add? The last five years have largely been a parade of genteel good taste. As Cher told Ms. Cosgrove, “I knew this could ruin a career — either momentarily or for a long time — because what you do at the Oscars, everyone sees.” Indeed, everyone saw Cher in her gaudy, belly-bearing Bob Mackie gown and feather headdress in 1986. Everyone laughed, too. But we’re still talking about her, and reminiscing about risk.