Oddly enough, never even heard of it, despite my love for Cornell Wilde. Here's what's funny. I read all the John Christopher I could find when I was a kid. Which meant The Tripods, Empty World and Fireball because the school library was run by a devout Mormon who hid all sci fi and fantasy as aggressively as she could. For some reason she disallowed me to do a report on Joan of Arc, despite the book being in her library, and forced me to do one on Brigham Young instead (this was not beneficial to my attitude towards the LDS Church in general or Mormons in particular). So I was this many years old when I not only found out about Death of Grass but that Fireball is the first in a trilogy. I wouldn't have even known about The Tripods if they hadn't been serialized in comic form in Boy's Life.
Man, I loved Empty World. Read it so many times. I grew up in the bush, with no neighbours, without electricity. Never travelling anywhere except the closest country towns. But every time I read Empty World it was as though I was travelling overseas. The way John Christopher described London I felt I knew it better than Melbourne. He also had a knack for writing protagonists who had a strong streak of bastardry in them. It was refreshing to read as a kid.