FANS of Celtic music have likely encountered Robert Plant or Alison Krause.

He is the bare-chested rock god who tucked Celtic and Nordic imagery into the heavy metal stomp of Led Zeppelin while exploring Irish music with side projects like AftoCelt Sound System.

She infused Irish fiddling into the bluegrass patchwork of her compositions. The two have collaborated now on Raising Sand. Crazy, I know, but this collection is as engaging and addictive as it is offbeat.

The musical collaboration of the year, Raising Sand is the sound of two iconic figures stepping out of their respective comfort zones and letting their instincts lead them across a brave new sonic landscape. Plant has said during interviews promoting the CD that he wanted to achieve "sexy Americana" with this record, and he has achieved that vibe thanks to sounds expertly honed by producer T Bone Burnett.

The sound can be best described as a swirl of early urban blues, spacious West Texas country, and the untapped potential of the folk-rock revolution.

Krause's funky fiddling weaves through "Please Read the Letter," a Plant composition, as Plant pleads and yelps. It is one of the only songs that either one of them wrote. For Raising Sand, they tackle an eclectic mix of songs penned by Tom Waits, Gene Clark, Sam Phillips, Townes Van Zandt, the Everly Brothers and Mel Tillis.

While I've always respected Krause's artistry, her voice was always a bit chirpy for my liking. On Raising Sand, she reveals layers of her vocals and phrasing that is sure to win her many new fans.

Over a musical bed that includes broken shells, scorched acoustic guitars and hushed xylophones, she haunts the spooky chords of Tom Wait's "Trampled Rose," and finds her inner rocker on "Gone, Gone, Gone." This track has a retro blues cool that allows Plant to stretch his vocal chords while Krause coos in the background.

In most places, Plant barely whispers over the pedal steel driven arrangements on tracks like "Killing the Blues." One of the most striking tracks is Van Zandt's "Nothin'," in which Plant whispers over drenched, howling guitar feedback that is tamed by Krause's sweet fiddle.

Music fans made this the number two CD in the country when it was released at the end of October, and critics have been lining up singing the praises of Raising Sand. Rightly so. This is easily one of the best things I heard all year.