It may seem strange and unfair to describe guitarist Roy Buchanan (1939-1988) as an underachiever. An uncommon master of the Fender Telecaster, Buchanan bonded gospel, blues and country dialects in a lashing-treble lightning-speed attack, with a melodic concentration that almost sounded like wild-animal singing. But Buchanan — a legendary sideman (he played on Dale Hawkins’ 1958 Chess single “My Babe”) and bar-band star in the Washington, D.C., area before he made his national debut in 1971 on a PBS documentary — was uneasy with fame and never embraced the role of frontman, spreading breathtaking solos across an uneven series of albums. Live: Amazing Grace (Powerhouse) is that career in miniature: snapshots of amazing twang pulled from a decade of gigs with shifting rhythm sections. It is also a blinding catalog of fire and invention — the clucking-chicken sass of 1983’s “Hot Cha,” the breathtaking sorrow of Buchanan’s instrumental prayer “The Messiah Will Come Again” on German TV in 1973, a free-form “Malaguena” from a second PBS show in 1972, an extended orgy of slash-and-curl on “Green Onions” from 1974 — that was never truly fulfilled and demands rediscovery.