The Pet Shop Boys’ underappreciated 2006 album ‘Fundamental’ is one of three being re-released in a deluxe expanded edition. (Photo courtesy the Karpel Group)

Reservoir wells for popular acts vary greatly in quality and quantity. There are almost always some little gems nestled in the outtakes, B-sides and deep album cuts the casual fans miss out on, but longtime fans know that Pet Shop Boys’ B-sides are often as essential as their singles and album cuts.

Sixteen years after a 2001 series of double-disc expanded editions, the veteran synthpop duo is once again mining their back catalog and this time they’re presenting fans with even more ear candy than before.

The first batch in a series that will eventually include all of their albums up through 2009’s “Yes” covers arguably the Pet Shop Boys’ three most underappreciated albums: “Nightlife” (1999), “Release” (2002) and “Fundamental” (2006). Once again Neil Tennant and Chris Lowe put the time and effort into carefully curated collections that other artists should emulate with their own archival releases. The first two are presented as three-CD sets including two full discs of “Further Listening” material, while “Fundamental” is two discs.

“Nightlife” (1999) is a glossy collection of well-crafted electronic dance pop, atmospheric and vast in sound but instilled with heart and genuine emotion. First single “I Don’t Know What You Want But I Can’t Give It Anymore” is one of their greatest studio triumphs, a true pop masterpiece of repressed rage simmering below an icy sheen of detachment as Tennant delivers barbed swipes at a cheating lover over Chris Lowe’s slow-building, immaculately arranged layers of string synths.

Other singles include “New York City Boy” and “You Only Tell Me You Love Me When I’m Drunk,” and there is a wealth of high quality bonus material like the excellent B-sides “Lies” and “The Ghost of Myself.” Studio outtakes like the tight disco-funk “Nightlife,” a track left off the album despite providing its title, are finally dusted off and getting a must-deserved moment in the sun.

Even more undervalued is the band’s next album, “Release” (2002). Johnny Marr of the Smiths’ fame contributes on guitar on an album that has a mellower vibe than typical for the duo, but the songwriting and arrangements are superb throughout. First single “Home and Dry” exudes the same kind of effusive love that flowed out of the speakers in palpable waves on their buoyant 1993 single “Liberation.”

The haunting allegory “Birthday Boy” is among the very finest recordings the duo has ever produced. Several highly commercial pop-oriented songs were dropped from the album to preserve its singular atmosphere, so the “Further Listening” discs will be sure to please fans who prefer the Pet Shop Boys higher-energy material. Highlights among the extras include “Sexy Northerner” and “I  Didn’t Get Where I Am Today,” a song that easily could have been a Top 10 single (in the U.K., that is — pop radio in the U.S. has basically ignored the duo since their last Top 40 hit “Domino Dancing” in 1988) if given the opportunity.

The third in the first batch of reissues is “Fundamental” (2006), produced by the great Trevor Horn. The duo returns to a more electronic sound but the album is still diverse, ranging from dance-floor bangers to poignant ballads. “Minimal” is the absolute highlight, a gleaming masterpiece of pop songcraft. The sharply political “Integral” could not have been more suited for our times today than if it had been specifically written about the last six months in America.

The album’s biggest hit, “I’m With Stupid,” is an artifact of the George W. Bush years, although the duo (and anyone else, for that matter) surely would have scoffed if told that a decade later a president would be elected that makes George W. Bush seem like a towering intellect. There are again plenty of extras to explore on the “Further Listening” discs, highlighted by outstanding non-album cuts like “The Resurrectionist” and “After the Event.”

The new Pet Shop Boys reissues should be cause for celebration among die-hard and casual fans alike, as they allow those who missed these great albums the first time around to discover them now, and the expanded “Further Listening” discs offer enough extra material to keep listeners well occupied until the next batch of arrives. They are a reminder of the seemingly limitless reservoir of amazing pop songs the Pet Shop Boys have relentlessly churned out over their remarkable 30-plus year career, with even more to come. Time to build that ultimate Pet Shop Boys playlist; one could hardly ask for a better soundtrack for summer.