The Everly Brothers--aka Don and Phil to fans with an intimate appreciation for them--seemed to exist almost as an apparition. Emerging within the formative era for young Baby Boomers during the blandly regimented '50s, they were a ubiquitous presence, clad in snug suits and skinny ties, hair neatly Brylcreemed, never raising their voices when they sang. The two prim-looking country boys with dark, curiously penetrating eyes and perfectly merged,...