The bluegrass joke asks how many bluegrass musicians it takes to replace a light bulb. The answer is five -- one to screw in the bulb and four to complain about going electric. The joke, of course, is at the expense of bluegrass' much cited traditionalism, including its insistence on acoustic instrumentation. However, if there is a singular conclusion to be drawn from Neil Rosenberg's "Bluegrass," it's that the music's traditionalism is more an artistic effect than a historical reality.

Rosenberg, who is an academic folklorist, has been a leading bluegrass historian for years, and it's not surprising that this book, 10 years in the writing, is the most authoritative and comprehensive account of the music yet written. Rosenberg also plays blugrass and organizes concerts and festivals, and perhaps that's why his book never loses sight of the music's essential character. Bluegrass continues to attract new fans and talented musicians because it's exciting.

The music's ability to dazzle an audience is wonderfully evoked in a remembrance of Cleo Davis, a member of Bill Monroe's Blue Grass Boys. Recalling the group's first appearance on the Grand Ole Opry, Davis says the audience "could not believe when we took off so fast and furious. Those people couldn't even think as fast as we played, I believe." After spending its first 90 pages tracing Monroe's development of the bluegrass style, Rosenberg's book also takes off fast and furious, weaving rich details into a variety of themes.

Rosenberg's central theme is that bluegrass' evolution can only be understood in terms of its complex historical relationships with country, rock and folk music. In fact, through the '50s, bluegrass was simply part of commercial country music and not until the late '50s was the term "bluegrass" even applied to the like-sounding music of Monroe, Flatt and Scruggs, the Stanley Brothers and others. It was really rock 'n' roll's impact on country music and country's subsequent turn toward the more urbane Nashville Sound that helped shape bluegrass' identity as a more pure, noncommercial and resolute tradition.

Rosenberg's rich understanding of bluegrass is colorfully underscored by his many accounts of the personal and creative struggles and bickering that characterize this music's history. Not surprisingly, many of these family squabbles involved Monroe, the music's stern and dignified patriarch, who would often shun acts that had imitated him (the Stanley Brothers), that had abandoned him (Flatt and Scruggs) or that simply didn't play or look right (any of the young "newgrass" acts).

The author also offers a complex reading of bluegrass' reflection in the national media, focusing particularly on the use of bluegrass as sound-track music in television's "The Beverly Hillbillies" and in movies like "Bonnie and Clyde" and "Deliverance." More than a few bluegrass artists were dismayed that the very exposure bringing their sound to the masses was also conveying images of redneck outlaws and backwoods hillbillies.

Despite its many flirtations with popular culture, by the late '60s bluegrass had begun to thrive as a small but vital world unto itself. Rosenberg discusses the growth of the bluegrass festivals, publications and small record companies that shaped this world. Considerable attention is devoted to Washington's progressive bluegrass scene, where acts like the Country Gentlemen and Seldom Scene helped forge a new and diverse audience -- young and old, northerner and southerner, redneck and hippie, middle class and blue collar.

If bluegrass music entered the '80s as a relatively stable entity, that hardly means the music is insulated from the constant changes in American popular music or that all of its artists want to remain untainted by commercial ambition or friction. Rosenberg knows that the recent commercial success of bluegrass-oriented artists like Ricky Skaggs and Emmylou Harris alone suggests that, in another decade, he will have some exciting new chapters to add to this dynamic history.