The easy-flowing Things Fall Apart made the Roots one of the most popular artists of alternative rap's second wave.

Anticipated nearly as much as it was delayed, the proper studio follow-up, Phrenology, finally appeared in late 2002, after much perfectionist tinkering by the band -- so much that the liner notes include recording dates (covering a span of two years) and, sometimes, histories for the individual tracks.

Coffeehouse music programmers beware: Phrenology is not Things Fall Apart redux; it's a challenging, hugely ambitious opus that's by turns brilliant and bewildering, as it strains to push the very sound of hip-hop into the future.

Despite a few gentler tracks (like the Nelly Furtado and Jill Scott guest spots), Phrenology is the hardest-hitting Roots album to date, partly because it's their most successful attempt to re-create their concert punch in the studio.

?uestlove's drums positively boom out of the speakers on the Talib Kweli duet "Rolling With Heat"; the fantastic, lean guitar groover "The Seed (2.0)" (with neo-soul auteur Cody ChesnuTT); and the opening section of "Water." The ten-minute "Water" is the album's centerpiece, a powerful look at former Roots MC Malik B.'s drug problems that morphs into a downright avant-garde sound collage.

Similarly, lead single "Break You Off," a neo-soul duet with Musiq, winds up in a melange of drum'n'bass programming and live strings.

If moves like those, or the speed-blur Bad Brains punk of "!!!!!!!," or the drum'n'bass backdrop of poet Amiri Baraka's "Something in the Way of Things (In Town)" can seem self-consciously eclectic, it's also true that Phrenology is one of those albums where the indulgences and far-out experiments make it that much more fascinating, whether they work or not.

Plus, slamming grooves like "Rock You," "Thought @ Work," and the aforementioned "The Seed (2.0)" keep things exciting and vital.

If this really is the future of hip-hop, then the sky is the limit.

[The two hidden bonus tracks are "Rhymes and Ammo," the Talib Kweli collaboration that appeared on Soundbombing, Vol.

3, and "Something to See," another techno-inflected jam.].