Since forming more than a decade ago, Guster has consistently been a great pop band. Five studio albums under their belts, they've rightfully amassed a wild fan base and a reputation for stellar live shows despite only modest radio play.

But the acoustic, college-rock mindset of their first three records shackled their sound -- and their songwriting. That's why, when they enlisted fourth member Joe Pisapia on guitar, banjo, bass, keyboards and harmonica in 2003, the result was "Keep It Together," their best release to date.

And that's why, three years later, they've topped it again with "Ganging Up on the Sun."

On "Keep It Together," Guster finally had the weapons of a real pop band: guitar, bass, drums and keys -- not to mention some of the best vocals this side of the Mississip'. This time around the band has all that, plus some experience with their arsenal. But rather than settle for matching their last release, they push into new territory with the exciting sound of a band in constant transition.

The Massachusetts trio-turned-quartet sticks to their strongest suit: Their angelic vocal harmonies are as accessible as ever, hooking listeners before they even realize how far the band's songwriting has come. Musically, the album expands in all directions, from the energetic rebellion of "New Underground" to the somber post-9/11 introspection of "Empire State."

But anyone familiar with Guster knows by now that radio-friendly melodies are par for the course. The truly impressive facet of "Ganging Up" is its verse.

This batch of songs is by far their most meditative and political. The album's title refers to a rebellion by the moon and stars, a struggle of the band members against an all-powerful being, described in the cheery piano bounce of "Manifest Destiny." This David and Goliath theme pervades many other songs, from a vow to "remain the honest soldier" in the sad-eyed prayer, "Lightning Rod," to Big Brother oppression in "The Beginning of the End," a jarring rock song with a howling guitar solo

Unique to Guster is that nearly all first-person references are "we" rather than "I." Rather than sounding like self-absorbed songwriters, the members use their familiar, likable personas to instill camaraderie in the listeners, uniting them all against "the sun."

Guster also takes to exploring their mortality. "Empire State," a hymnal whispered from the rubble of the World Trade Center, tells of slow, solitary rebuilding. Miller heaves his futility with the line "Been talking to Jesus; he's not talking to me," but the sentiment is likely more symbolic than religious.

The band delves into an afterlife scenario in "Ruby Falls," a scorching, majestic seven-minute plea to their maker worthy of a spot on Pink Floyd's "Dark Side of the Moon." "Judge, can you look the other way?" wails Miller over harrowing gospel choir and organ chords. He is broken, having reached the end of the road and left at the mercy of the omniscient. What's more, the song shares its name with America's largest underground waterfall, a Tennessee attraction that eerily looks like Lucifer's domain.

All this soul-searching dissolves into uncertainty on the album's final track, "Hang On." With lyrics like "If we fell inside a forest, would it make a sound / It doesn't seem there's anyone around," the band writes like they're confused, floating in the ether without control of they're own fate.

"Ganging Up on the Sun" is their most ambitious album yet, and nearly every risk pays off. (The only misstep is the single, "One Man Wrecking Machine," where the unlikable narrator is hard to swallow from a famously friendly group.) Guster has matured musically without losing the pop sense that graces their melodies, and while their songs may struggle with uncertain mortality, the band's direction has never sounded more alive.