The Athenaeum must be one of the loveliest hidden delights of Boston. I went there over the weekend while I was in town recording an episode of VoiceBox about street musicians with my friend, Sophia, who is a member. I doubt I would have even noticed the inconspicuous stone building wedged between other more imposing edifices in downtown Boston had I not been in the company of someone in the know. I'm so glad we went…