We have long known that there is a division between literary fiction and the mass market, but it says something about the fragmentation of pop music that there is now some kind of musical equivalent. The Billboard charts of top-selling LPs in the month of July, 1971, included “Sticky Fingers,” by the Rolling Stones; “What’s Going On,” by Marvin Gaye; “4 Way Street,” a live double album by Crosby, Stills, Nash & Young; “Aretha Live at Fillmore West,” by Aretha Franklin; and “Mud Slide Slim and the Blue Horizon,” by James Taylor. You can imagine that at the time even the most curmudgeonly critics might have found it in themselves to gush over at least a couple of those. Now, however, in addition to the self-explanatory top-seller charts, we have myriad other lists, from MP3 downloads to top alternative albums to top college, whatever that may mean. (These lists may well have been born of the music critics’ despair at popular taste.) There is “literary,” critically approved pop — Lucinda Williams, say, or Wilco, or Nick Cave, none of whom trouble the Billboard statisticians much — and the MTV-driven hard rock, rap, and R. & B. that you can find at the front of your local HMV. Some might argue that the critics who wrote about Marvin and Aretha thirty years ago are the very same people who rave about Lucinda Williams today, and they’d have a point: rock critics now seem to have tenure, like senior faculty, which could explain why current youth-targeted music seems relatively unexamined.