When Neil Young enters what his authorized biographer calls signature trance-outs, he can play endless, indecipherable music that grows enveloping almost in spite of itself. The author of ''Shakey,'' a mammoth portrait of the artist and lively exhumation of rock 'n' roll history, has shaped his book the same way.

Yes, nearly 800 pages of text -- including long interview passages immortalizing Mr. Young's every ''gonna,'' ''wanna'' and ''kinda'' -- make this sound like a flabby undertaking. On the other hand, Jimmy McDonough's sprawling book exerts a deep, stubborn fascination. Its view of the man once nicknamed Ancient History Up Close (by the 80's group Devo) reflects the mutable pop culture in which Mr. Young has remained afloat for nearly 40 years, evolving unpredictably and single-mindedly without a whiff of compromise.