The Sweet Valley High series- only up to number 15, because they didn't bother translating anymore. I was ten years old! I must admit that it was the first time I was introduced to themes like sex, drugs and the teenage world. They were rather cheap and I could afford four of them out of my allowance. At the same time I started reading the Fear Street series- they were priced the same.

I still own them all. I kept them for my little sister, who wouldn't touch a book unless at gunpoint, so they'll probably end up in a box in the attic any day.