In this irresistible guide to hillbilly's most haunting songs, Cantwell and Friskics-Warren's prose often delivers the loose thrill and emotional immediacy of the music itself. They describe the ''itchy guitar lick'' on Bobbie Gentry's ''Ode to Billie Joe'' as ''the sonic equivalent of someone picking a scab or scratching a mosquito bite.'' And Jerry Lee Lewis' ''Great Balls of Fire'' is so randy that ''the Killer could be speaking in tongues and it would still sound like lust.'' Small irritants abound: Dolly Parton hails from the Smoky Mountains, not the Blue Ridge, and ''Don't Be Cruel'' can't possibly be the first Nashville Sound recording. Still, this is required reading for anyone who's ever swooned at the sound of a steel guitar.