Basically it's how I ate when I was a kid and throughout my teens: 3 decent meals per day and no snacks or sweets, apart from having a little treat on S days. S days are Saturdays, Sundays and Special days such as your birthday, family celebrations, Christmas and Easter. As long as there are no more than 2 "Special" days in a month, you're sticking to the diet.
And that's it! That's all you need to know really. You don't need to buy the book, I've told you all the rules! But the book is worth having because it explains everything so clearly. You don't have to give up any of your favourite foods, just eat less of them if they're sweet or snacks. Nothing is forbidden, you're just reducing the amount you eat by focusing on when you eat. The book also has lots of useful advice on forming good habits rather than trying to rely on willpower.
Most diet books spend 90% of the book explaining the diet and just the final chapter on telling you how to maintain your weight when you've lost as much as you need. They explain how to diet, but they don't tell you how to establish a healthy relationship with food for the rest of your life. The No S Diet is basically a maintenance diet. You can live on it for the rest of your life without suffering from a sense of deprivation. You won't lose weight fast. I've been following it for just over a week and have possibly lost 1lb (the author has important things to say about the unreliability of scales!). When I've dieted previously, I've lost a steady 2lbs per week. But I know that when I was young, this is how I ate and I stayed slim. Somehow, in recent years I've lost touch with those good habits. At my age, I can't afford to just keep piling on the pounds, and this diet is simple enough to require little conscious thought to keep to the rules and lenient enough that I can enjoy my plain food through the week and look forward to guilt-free chocolate and cake at the weekend.