Composer Max Steiner, whose scores for “King Kong,” “Gone With the Wind” and “Casablanca” placed him in the movie-music pantheon, isn’t much discussed today. He seems to belong to that old-school, pre-synthesizer world of orchestral scoring from the ’30s, ’40s and ’50s.

But as author Steven C. Smith points out in his engrossing new biography of the three-time Oscar winner, “Music by Max Steiner” (Oxford University Press), the Austrian wunderkind pioneered the art of film scoring and ranks as “Hollywood’s most influential composer.”

His music essentially saved RKO’s “King Kong,” the 1933 giant-ape-wrecks-Manhattan fantasy, forcefully demonstrating the power of dramatic underscore to create mood, propel the action and provide emotional support (and disproving the widely held studio-executive theory that audiences of the time would “wonder where the music came from”).

Steiner went on to score some 300 films over a 35-year career, mostly for RKO and Warner Bros., although his most famous work was done on loan-out to independent producer David O. Selznick: 1939’s “Gone With the Wind,” which Variety astutely called “a symphonic tone poem of a score that beautifully captured the elegance, sentimentality, pathos and gallantry of the period.”

He wrote three hours of music in 12 weeks, including the immortal “Tara” theme that became synonymous with Hollywood (and, New Yorkers will recall, became the opening music for WOR’s nightly “Million Dollar Movie”).

And while Smith’s book concentrates on his dramatic scores, he also discusses in detail Steiner’s critical role as music director of the first five Fred Astaire-Ginger Rogers musicals, collaborating not only with Astaire on the dance sequences but also with such songwriting giants as Irving Berlin (“Top Hat”) and Jerome Kern (“Roberta”).

It was like going home for Steiner, whose musical career began in Vienna — writing operettas at the age of 18 — but shifted to London and New York, where he orchestrated and conducted for the theater (including the Gershwin musical “Lady Be Good” in 1924).

Summoned to Hollywood in 1930, he soon began experimenting with dramatic music in such films as “Cimarron” and “Symphony of Six Million”; perfecting the use of the click track (a device used in cartoons to synchronize music and image), still in use today; and adapting Richard Wagner’s concept of the leitmotif — themes for characters, places and ideas — for the big screen.

Even on troublesome projects, the dedicated composer came through. “Max Steiner hated ‘As Time Goes By,'” Smith informs us about the centerpiece song of “Casablanca.” But it was written into the script, so Steiner turned it into a love theme for Humphrey Bogart and Ingrid Bergman and helped to turn their complex relationship into an indelible piece of Hollywood history.

Romance, in fact, was a Steiner specialty, especially on more than 20 Bette Davis films, including everything from “Jezebel” and “Dark Victory” to “Of Human Bondage” and “The Letter.” One of his three Oscar winners was Davis’ “Now, Voyager,” the 1942 classic whose heartfelt theme became one of the first hit songs adapted from a film score (“It Can’t Be Wrong”).

Another Oscar winner was “The Informer,” one of three John Ford films with Steiner scores, the last of which, 1956’s “The Searchers.” is now considered one of the greatest Westerns ever made. Steiner not only underscored John Wayne, he was a frequent accompanist for Humphrey Bogart (private-eye music for “The Big Sleep,” a south-of-the-border ambiance for “The Treasure of the Sierra Madre,” a Naval march for “The Caine Mutiny”).

By the late 1950s Steiner was widely considered out of date and out of fashion. Yet amazingly, as Smith recounts, the 71-year-old Steiner rebounded by penning a pop hit. His young-lovers theme for “A Summer Place,” a widely panned Sandra Dee-Troy Donahue drama, hit No. 1 on the charts in 1960 and stayed there for nine weeks.

It was a sad decline from that height. Steiner’s son Ronnie committed suicide in 1962; he was slowly going blind, found assignments harder to come by, and finally retired in 1965. Yet he remained quick-witted, funny and sarcastic to the end, Smith reports. As “King Kong” producer Merian C. Cooper declared at Steiner’s 1971 funeral: “His music will live on like those men of Vienna whom he followed: Mozart, Beethoven, the Strausses, all of them. Maxie’s music has a true drama, screen drama, music as immortal as anything will ever be.”

Today, with nearly all his movies available via either streaming or DVD, Steiner’s work, writes Smith, “transports audiences into worlds larger than life in their heightened emotion, yet instantly relatable in their expressions of joy, pain, and romantic fulfillment.”

(To read an interview with Smith about his Steiner biography, click here.)