Another one agreeing with Breadcrumbs here! I think he was almost single-handedly responsible for getting many, many young people (especially young men) interested in cooking for pleasure back in the late nineties, using fresh, simple ingredients and unfussy 'uncheffy' recipes.
He not only demystified cooking for so many, but he made it fashionable and cool. He inspired people with the confidence to entertain at home, instead of feeling overwhelmed or intimidated by the prospect.
He has also encouraged people to have fun with food, always with the emphasis on simplicity and good quality fresh produce. He has made vegetables and salads sexy, and his food presentation style is (while a tad messy for my liking at times) unpretentious and therefore completely achievable by all. I doubt we'd be seeing the huge unprecedented obsession with cooking, baking and vegetable growing that has hit the UK in recent years if JO had not played his part so well all through the noughties.
I think the country is also indebted to him for bringing about a lasting change in the quality of school meals. That programme alone will also have caused many to question the quality of the processed foods they routinely fed their families.
I love his books, but I haven't bought all of the more recent ones as I find them a little basic and/or repetitive for my needs now. However I think there is no-one better to help you get started and easily inspired as a beginner.