Loudon Wainwright III is just a little famous, but he’s been a little famous for a really long time. He was in M*A*S*H* on TV. He’s Rufus and Martha Wainwright’s dad. He was in Judd Apatow’s Undeclared and The 40-Year-Old-Virgin. And he writes folk songs that will make you weep. Seriously. “Motel Blues” is just about the sweetest song about sex with a groupie that you will ever hear, and “Daughter” -- a song Apatow used in the otherwise just-OK movie Knocked Up -- deals with the subtle heartbreak of growing up and makes me tear up every time I hear it. He has the narrative command of Pete Seeger, the easy warmth of Cat Stevens, and from what I’ve seen of his performances on YouTube, he’s even better in person. His new box set includes four discs’ worth of tracks from his 40-year career, plus a DVD with tons of footage from documentaries, BBC specials, appearances on Austin City Limits, Saturday Night Live and more.