There are some box sets that seem like overkill, beyond the pale for all but the very most hardcore fans, and others - a little more obvious in their justification - that never achieve much currency beyond the ranks of the serious fans and as easy Christmas ideas for their relatives. And then there are the ones that, based on the sheer credibility of the artists involved - Eric Clapton, Elvis Presley, the Beatles, Bob Dylan, Frank Sinatra - become practically standard-issue for any serious music listener; you expect to find at least one, and more likely two of them on a lot of shelves. The Classic Years 1927-1940 ought to fit into the latter category, despite the fact that Blind Willie McTell never had a hit record in a recording career lasting nearly 30 years - he also didn't make Rolling Stone's list of the 100 greatest guitarists of the twentieth century, even though he could play circles about three-fourths of those who did…