The terror of Day of the Triffids doesn't exactly come from the threat of the titular giant, slow moving carniverous plants that serve as the monsters in this YA-SF classic; the most horrifying part of the book is the idea that the world could change completely and irreversibly in an instant. It's a fear grounded in our psyches today and surely it was on the mind of John Wyndham when he wrote this seminal novel in 1951. At that time it was the threat of war from shady unknown enemies and while the names have changed today, the fear really hasn't–which makes Triffids as relevant as ever.
The novel hardly shows its age and was clearly an influence on the new classic 28 Days Later. I also recommended it to my dad who works with similar themes in his awesome upcoming graphic novel, In the Small (pre order now!)?The believable characters and thought provoking story gnaws at you days after you've read it as you wonder what you would do in the same situation. I won't give too much plot away but as the disaster sets in, it's human characters in their desperate bids for survival, not monsters, that pose the most threat.
The book was made into a 1962 film and a miniseries (see this week's TV pick) in 1981, when again there was the threat of annihilation. Triffids is not the only work of Wyndham's to be adapted for film. He wrote The Midwich Cuckoos, the basis for the unforgettable Village of the Damned; and Chocky, which was made into a famous BBC show (it's next on my wish list).