Soul music reached a peak of sophistication in the early 1970s. It drew boldly from rock, jazz and symphonic works as well as the genre’s bedrock of gospel and blues. Lyrics addressed social ills with a boldness that would have been unthinkable even in the late ‘60s.

Kenny Gamble and Leon Huff were two of the most prestigious and prolific creators of the period. The pair wrote and produced hits for the O’Jays, Harold Melvin & the Blue Notes, Billy Paul, The Three Degrees and many more. They released them on their own Philadelphia International label, which for a few years had the Midas touch.

Two recently released compilations cover PI’s glory years - one has the biggest hits, the other songs that fell between the cracks.

“The Sound of Philadelphia: Gamble & Huff’s Greatest Hits” is a 14-track best-of that reads like a mid-’70s Top 10 chart: “Love Train,” “Back Stabbers,” “If You Don’t Know Me By Now,” “T.S.O.P.,” “When Will I See You Again,” “Ain’t No Stoppin’ Us Now,” “You’ll Never Find Another Love Like Mine” - it’s a can’t miss collection.

But it just skims the surface of Gamble-Huff’s catalog and offers no essays or background information. Fine if you’re on a budget, but the O’Jays, Paul, Melvin and more put out great albums of their own, and 1997’s “The Philly Sound 1966-1976: Kenny Gamble And Leon Huff” three-cd box set offers a far more comprehensive look.

Even owners of that now-out of print release will want to seek out “Conquer the World: The Lost Soul of Philadelphia International Records.”

The disc contains 16 tracks that might stump even the most avid of soul fanatics. Admittedly there’s nothing here that would give Gamble and Huff’s best a run for the money, but there are plenty of gems, including Bunny Sigler “Theme for “Five Fingers of Death"," Ruth McFadden’s assertive “Ghetto Woman Parts 1 & 2” and the atypical comedy track “Ruby’s Surprise Party” by, naturally, Ruby & the Party Gang.

Add to that Sigler and Dee Dee Sharp’s “Conquer the World Together,” and Yellow Sunshine’s eponymous slice of soul psychedelia and you have a fine, fun collection of rare R&B.