"The book, Riddle of the Sands, was talked about a lot."
Mostly be me, I think. I said it had been voted one of the best spy novels of the 20th century, although it isn't exactly an easy read.
The film is excellent - one of the rare examples where what appears on the screen is as good as the book. It's worth watching for the look of it alone - it's beautifully shot, and a Jenny Agutter is easy on the eye. If you like sailing-boats and a good spy story it's a must-see.