Matthew and Eleanor Friedberger, the brother-sister duo known as Fiery Furnaces, have become the standard bearers of underground progressive rock by reviving the idea that albums can be complete, integrated pop works unto themselves. In this age of institutionalized short attention spans and the iPod Shuffle, such an approach might sound regressive, if not downright quaint.

The Furnaces' second album, "Blueberry Boat," released last year on Rough Trade, was a true test case: an ambitious work of art whose derived pleasures greatly depend on how willing you are to give yourself over to the album form. Critics were sharply divided, with some claiming that listeners who didn't lend it the full attention it deserved were simply lazy, while others dismissed it as turgid self-indulgence.

But while the cuts on the band's first album, "Gallowsbird's Bark," are more conventionally structured, the manic energy and surfeit of musical ideas they contained made it clear that the Furnaces were interested in much more than songs. "Blueberry Boat," which is over 70 minutes long and is essentially an extended suite of disparate and sometimes repeated melodic and lyrical ideas, represents the siblings' sensibility more faithfully.