I loved the Secret Seven series when I was younger. I used to save up all my pocket money to buy them and eventually I had all of them. If you're part of a brother/sister relationship and you don't always get on, read about Susie and Jack as they squabble. It's great fun.
I'd recommend the Secret Seven to all kids of all ages. They were always full of adventure and made me wish that where I lived would be more adventurous. Terrific books to capture the imagination.