The last time Justin Townes Earle played Louisville, he told a story about how Guy Clark advised him that, as a songwriter, you have to be careful about every song you write, because you never know which one people will use to define you for years to come. (Not that every aspect of that is necessarily problematic). In Vanessa Carlton’s case, the instant association is “A Thousand Miles,” but, as with any artist, you should dig deeper — there’s a discography full of solid singer/songwriter material that gets sharper and darker along the way.