There is an article about the Clash at Slate. Here's a bit of it:
On the Clash's London Calling, the pink-and-green lettering that frames Paul Simonon—red London down the Y axis, green Calling across the X—mimics the "first" rock album, Elvis Presley's 1956 debut. This is not an homage. Where Elvis was holding up his acoustic guitar, shouting blues to the rafters, Simonon has split his legs wide, the better to drive the head of a Fender Precision into the stage at the old Palladium. The Clash were built to smash twilit idols—"no Elvis, no Beatles, no Rolling Stones" had been the band's rallying cry since 1977. The working title for London Calling was "The Last Testament," and it was supposed to stand, according to the band's manager, as the "last rock and roll record."
...Why were the Clash so well-positioned to take punk rock beyond punk rock? This will strike some ears as heresy, but the first reason is simple: The Clash weren't a punk rock band. Joe Strummer had fronted a group called the 101ers, a pub-rock outfit more in the tradition of Dr. Feelgood or Brinsley Schwartz than the New York Dolls or the Stooges. (Before that, he passed himself off as a folkie, demanding to be known simply as "Woody.") His hokey stage bravado, which so blew away his new chum Mick Jones, was based on Strummer's recent study of Bruce Springsteen, who had been headlining at the Hammersmith Odeon. For his part, Mick Jones was an avowed punk, the better to spite his own sweet disposition. Before they became famous, Jones and Chrissie Hynde hung out and wrote music together. She later remembered his creations as "rather dippy love songs."
...Early in the Clash's history, Strummer tossed out some snotty asides from the stage about how A levels (the "Advanced" standardized test often necessary for university entrance in Britain) were a conspiracy, when a girl in the audience shouted, "Your drummer's got them," promptly silencing Strummer, who, as it turns out, had at least one himself. For his part, Mick Jones passed five O levels (the standardized tests British children take in their mid-teens) and attended a demanding grammar school from which he could reasonably have expected a white-collar future.
I've never really understood why people love the Clash so much. I have always felt them to be a little on the phony side (which is why I cherry-picked quotes to support this idea), and their music has never moved me. I have to admit that I haven't listened to the Clash all that much, so maybe there is something to them that I'm just getting.