By the mid-90s, I'd pretty much lost touch with American alt rock. Grunge had been well and truly co-opted, while all the papers wanted to write about over here was Britpop. As a result, Unwound – perhaps the last great band in the original post-hardcore lineage – nearly passed me by completely. But I read a review of 2001's Leaves Turn Inside You, and was intrigued enough to pick up a cheap promo. After a few plays, it dawned on me that it was one of the most brilliant albums I'd ever heard. Unfortunately, it was also to be their last.

Influenced by both Sonic Youth and Fugazi, Unwound had reached a reasonable level of recognition (in the US at least) and some commercial success with the release of Repetition in 1996. Having achieved their relatively modest ambitions, they were faced with the problem of what to do next. Empire is the final release in the Numero Group's Unwound reissue programme, and collects all of their output post-Repetition. It also includes an excellent long-form essay on this period of the band's history, which captures a familiar tale of slow disintegration brought on by splintering relations, creative struggles and the war of attrition against sanity known as touring.

Their follow-up to Repetition was 1998's Challenge For A Civilized Society, a flawed but fascinating document of a band in transition. All the virtues that had got Unwound to where they were are evident on opening track 'Data': Justin Trosper's alternatively fiery/needling guitar style and angry Thurston Moore-esque vocals; Vern Rumsey's muscular, melodic bass undercarriage; and Sara Lund's explosive, almost jazzy drumming filling out the trio's sound. But there's something audibly eating away at them, the mood of the song shifting restlessly from attack to confusion to resolution. 'Laugh Track' features another great twisting riff, like a double-speed Slint, but again, it sounds like a song at war with itself, tuneful but uncomfortably exposed.

This sense of panic, with occasional lapses into inertia, means that Challenge… isn't always an easy listen. The spiky, coiled fury of 'Meet The Plastics' and 'The World Is Flat' is music as self-defence as much as self-expression, and while tracks such as 'Side Effects Of Being Tired' and 'Lifetime Achievement Award' show a willingness to go in a more experimental direction, they're exercises in pursuing inspiration rather than finding it. Yet there are hints of what's to come, particularly on (the somewhat archly titled) 'Sonata For Loudspeakers', its thicker guitar sound, straighter beat and languorous trumpet producing something that's both looser and more purposeful.

Challenge… may not always have had the courage of its convictions, but it showed that the band were intent on stretching themselves. Their post-hardcore heritage was still in play, but the angsty modern prog of Radiohead was also exerting a strong influence (check out some of those song titles above). Just as Yorke & co sought to re-invent their sound for the 21st century, Unwound went through a similar process, building their own studio to give themselves the time and space to re-think their approach to music-making. Kid A might have got all the plaudits, but Leaves Turn Inside You is in many ways just as radical, and has arguably better stood the test of time.

Released as a double album in April 2001, Leaves… is both beautifully austere and thrillingly expansive. Despite the air of winteriness and mystery that permeates the music, there's a new-found confidence and go for broke-ness on display. Slowly flowering out of an extended fanfare of e-bowed notes, opening track 'We Invent You' immediately draws a line in the sand with the past. Its delicate guitar, big drums and blissful, breathy vocals invite the listener in, then envelop them in waves of cello and Mellotron. 'Look A Ghost' and 'December' twist and buck like before, but the sound's lighter now and all the parts fit perfectly together. Things get really interesting on the Wire-meets-King Crimson three part epic of 'Terminus', a brilliant exercise in tension and release. Trosper's steely, cyclical guitar figures slowly escalate, pushing Lund's relentless percussive clatter into the background like an army battering at the gates of a castle. Guitar, bass and cello pick out a taut, pizzicato rhythm before shooting skywards, leaving behind a vapour trail of haunted strings. It ends with a final section of post-rock finger picking and floating, spectral keys. It is breathtakingly audacious.

The album is packed with such unexpected treats. 'Demons Sing Love Songs' is harpsichord-driven psych pop with a dark undertow, its murmured verses leading to a gloriously hazy chorus that nevertheless impels you to sing along. 'One Lick Less' has the same dronic, gossamer drift as MBV, and anticipates the nu-gaze and bedroom psych scenes of recent years. Both 'October All Over' and 'Summer Freeze' suggest that Spiderland also figured strongly in their psychic arsenal, but theirs is a spacier, more luminous take on that album's claustrophobic sound.

Just when you think you've got a handle on the album, along comes the astonishing 'Radio Gra'. It starts with a pre-war jazz loop and an oddly playful Polish radio broadcast, creating a wormhole from the past through which a Mellotron crashes, its ghostly strings soaring and weaving around the song's minor key guitar picking and wordless vocal croon. It's one of those tracks that's somehow indescribably moving. And then we get the album's second epic, 'Below The Salt', a stately processional anchored by a repeated bass figure and coloured by spare, impressionistic piano. It slowly unfurls, its sombre guitar and hypnogogic vocal creating an almost Floyd-like vibe.

On its release, Leaves Turn Inside You wrong-footed many of Unwound's existing fans without gaining the attention of a new audience. And as inter-band tensions became intolerable and the tour to promote the album ran headlong into the terror of 9/11, Unwound decided they just couldn't go on. But at least we've still got the music. A masterpiece in its own right, Leaves… deserves to take its place alongside Zen Arcade and Daydream Nation as one of the great post-hardcore doubles.