I read this book years ago at school, or some of it, and I'm wondering what it was.
A space mission (to somewhere in the Solar System - I'm thinking Venus) had unwittingly brought back spores of a terrible grey fungus. The fungus spread at a terrifying rate, it ate a whole town, smothering everything in a thick grey blanket, and the town was sealed off to prevent it spreading.
The main characters were taken to see this place, all wearing spacesuits to keep the fungus out. The suits were made to be fireproof and before they left the dead town, the guards ran flamethrowers over them as that was the only sure way of killing the spores. I remember that while they had their helmets on they could only communicate by writing on slates and holding them up.
I think the main characters were astronauts who were being asked to go back to Venus (if it was Venus) and find out what kept the fungus from overrunning Venus so they could use that to get rid of it on Earth.
There was, I think, a sequel where the same astronauts went to Mars and there encountered disembodied minds, the last remnants of the Martians, and it was very hard for them to outwit these creatures because they could read their minds and know what they were planning before they started.