Fifty years ago this month, Peggy Lee recorded “Fever.” The song was neither her biggest hit nor her own composition, but today it is not just her signature, but one of the most iconic pop records ever made. Now, 50 years later, with a counterintuitiveness Lee would have approved of, her daughter, Nikki Foster, and granddaughter, Holly Foster Wells (working in conjunction with Collectors’ Choice Music), are commemorating the golden anniversary of one of the best-known songs of all time by releasing some of the least-known music of Lee’s career.

Lee (1920-2002) is generally regarded as one of the foremost interpreters of the Great American Songbook, a crucial hit maker of the same vintage as Frank Sinatra, Ella Fitzgerald, and Nat King Cole. Like most of her contemporaries, Lee started as a big-band vocalist (with Benny Goodman) before becoming a solo headliner. But there were several factors that made her unique: While she might have had less in the way of pure vocal chops than the others of her echelon, she had a greater sympathy for the blues (an unusual talent for a woman of Scandinavian descent from North Dakota) as well as a gift for songwriting equalled by few singers.

Much of the material being issued Tuesday by Collectors’ Choice (excepting “All Aglow Again,” which consists primarily of hits) is aimed at hard-core Peggy Lee completists. But any fan of popular music is better off being a hard-core Peggy Lee completist. As with Sinatra and Billie Holiday, Lee has much to teach us even in her weaker moments.

The double disc “Lost ’40s and ’50s Capitol Masters” essentially provides what the title promises, though “unheard” may be a better word. The set features 39 tracks, recorded between 1944 and 1951, that have never seen the underside of a CD, 12 of which were never released at all. Lee is all over the map here, from the Mississippi Delta to Nashville to a boulevard café on the Champs Élysées, where the Gypsies used to play.

On a characteristically jazzy treatment of the traditional spiritual “Swing Low Sweet Chariot,” she thrillingly takes the command to “swing” literally. But the disc also features a surprising number of country-centric singles, including her self-penned, hoedown-style square dance “Neon Signs,” a stylish and decidedly non-hillbilly treatment of Spade Cooley’s country hit, “Shame on You.” The real treasures are a group of previously unheard, top-drawer recordings from the mid-’40s, such as “Music, Maestro, Please,” “I’ve Had My Moments,” and “A Cottage for Sale.” It is unimaginable why Capitol Records would have kept these gems in the can for almost 65 years; they are considerably better than a lot of the contempo songs the label did release during these years.

In 1958, when Lee’s “Fever” hit the Billboard charts, Capitol did not rush to the stores with a “Fever” album — that idea only became standard in the record business a few years later. Instead, the label used “Fever” as the lead-off track on a budget LP called “All Aglow Again,” which turned out to be a tidy collection of her hits for the label.

That album is part of the new Collectors’ Choice gathering and, like the rest, it’s reissued with marvelous sound, packaging, and bonus tracks. (In fact, you could say that “The Lost ’40s and ’50s Capitol Masters” consists entirely of bonus tracks.) In the late ’50s and early ’60s — the glory years of her career — Lee was juggling traditions, keeping current with Elvis Presley and Ray Charles (as on “Hallelujah, I Love Him So”) whilst continuing to record standards and new show tunes (“Where Do I Go From Here”).

A decade later, Lee enjoyed a brief but important resurgence as a hit maker with “Is That All There Is,” released in a period when most of her colleagues had given up the idea of getting back into jukeboxes. Lee was still turning out two or three albums a year, four of which are now appearing on two CDs, “Make It with You”/”Where Did They Go?” and “Then Was Then, Now Is Now!”/”Bridge Over Troubled Water.” Even though she cannily inserted Arthur Schwartz’s “I See Your Face Before Me” (with a wailing guitar accompaniment) in the midst of the latter album, originally released in 1970, by this point the bulk of the material Capitol was permitting her to record was hardly worthy of her: There are muzak-y treatments of contemporary pop-rock hits such as “My Sweet Lord” and “Raindrops Keep Falling on My Head,” which seem a dreadful comedown for the chanteuse who once sang “Stormy Weather.” Somehow, the wishy-washy arrangements make Stephen Sondheim’s “Losing My Mind” sound as toothless and insipid as Andrew Lloyd-Webber’s “I Don’t Know How To Love Him” — not a good thing.

Clearly, Lee’s heart was not in a lot of the material Capitol was putting in front of her in the post-psychedelic era. Some of it is more interesting than good: There are no fewer than three “Is That All There Is” sequels — “Where Did They Go,” “One More Ride on the Merry Go Round,” and “You’ll Remember Me,” all of which are somewhat surreal, Fellini-esque waltzes with circus allusions and off-kilter calliopes. “Merry Go Round” even utilizes sound effects and dialogue depicting a carnival atmosphere.

Nevertheless, there are more than enough buried gems to justify the purchase. “Everybody Has the Right To Be Wrong” finds Lee reimagining a contemporary show tune as a cha-cha-cha (à la “Latin a La Lee”); “This Could Be the Start of Something Big” is a previously unheard and irresistible swinger in a swashbuckling Erroll Garner tempo, with bongos and claves. There are also all manner of fresh Peggy permutations of the blues, such as B.B. King’s “The Thrill Is Gone” and Willie Dixon’s “Seventh Son.”

But the truest treasure of the series is “I Was Born in Love With You,” which appears on one of Lee’s least known LPs, the aptly titled “Where Did They Go?” The track is one of three early songs by Michel Legrand, with lyricists Alan and Marilyn Bergman (along with “Pieces of Dreams” and “What Are You Doing The Rest Of Your Life?”). It’s a stunning performance of an underappreciated prize of a song, and a perfect example of Lee’s maximal-minimalism, in which she extracts acres of meaning with as little movement as possible, using miniscule details to convey epic ideas. She seems, almost passively, to let the song itself sweep her upward into a maelstrom of emotion and understated drama. She lets the song sing her.

wfriedwald@nysun.com