This is a very upsetting, terrible loss. His books were stunning, not so much about spying but about human relations. His best well known books were Tinker, Tailor and Smiley’s People, which I have read two or three times. His big break came with The Spy Who Came in From the Cold, which broke the mold on how spies until that time (James Bond and progeny) had been portrayed. Some of his lesser books were very good too. A Small Town in Germany comes to mind as does The Looking Glass War. A Small Town came out after Spy and wasn’t well received at the time but has come to be appreciated in intervening years.
For those who haven’t read him but might be interested in trying, I recommend The Spy Who Came in From the Cold. The movie with Richard Burton closely follows the book so if you liked the movie you might look the book. I also recommend the BBC series of Tinker, Tailor, not the movie (which isn’t very good) but the six hour series made in 1980. Unfortunately, it’s not on Amazon but on Acorn TV.
RIP Mr. Cornwell (the nom de plume of le Carre).