''Aja'' was the album that made Steely Dan a commercial force on the order of contemporaries like Fleetwood Mac, the Eagles and Chicago. A double-platinum, Grammy-winning bestseller, it lingered on the Billboard charts for more than a year and spawned three hit singles. Odd, then, that its creators saw it as an ''ambitious, extended'' work, the antithesis of their anti-rock, anti-band, anti-glamour aesthetic. Populated by thirty-five mostly jazz session players, ''Aja'' served up pre-war song forms, mixed meters and extended solos to a generation whose idea of pop daring was Paul letting Linda sing lead once in a while. And, impossibly, it sold. Including an in-depth interview with Donald Fagen, this book paints a detailed picture of the making of a masterpiece.