When it comes to the jam band scene, people either know everything or nothing at all. A good introduction to this subculture is “Heads,” the new book by journalist and critic Jesse Jarnow. Steve talked to Jesse about the book, the latest incarnation of the Grateful Dead with John Mayer, Dead and Co., and how he broke up Phish in the early ’00s. Then (at 49:29) Steve talks with one of his favorite singer-songwriters, Todd Snider, about why he joined a jam band, Hard Working Americans, with members of Widespread Panic and the Chris Robinson Brotherhood. Todd also shares the story about the time he spent in rehab with Phil Hartman’s wife before her and her husband’s tragic death.