Few figures were as instrumental to the British folk music revival as John Renbourn, whose guitar style incorporated influences from Celtic, jazz, pop, R&B, and beyond. As one-fifth of Pentangle, he and his bandmates expanded the possibilities of a folk-infused combo with odd time signatures, lengthy instrumental passages, and a repertoire that spanned decades, genres, and forms. But even before forming Pentangle, Renbourn had stunned audiences and listeners with his unique approach to the guitar, eventually earning the nickname “the stoned Julian Bream of folk guitar” from Zig Zag founder and historian…