Oh boy Fleur, it hasn't even dented the collection!

15yo girl - maybe the Meg Rosoff "just in case", the Kim Wilkins "Witchsong" and the Rhian Tracey "The bad girls' club". I honestly can't tell you the content though, as I haven't read either, but they seem to sit at that age.

13yo boy - "Eagle Strike" by Anthony Horowitz sits with James Patterson/Matthew Riley but is a little younger. Patterson and Riley are aimed at adults, but I know plenty of teens who have read them and loved them.

8yo boy - I'd probably go with "Pirates eat porridge", "The wish list" and "Artemis Fowl". But again, depends on his reading level - Artemis Fowl may be too hard, but he will grow into it.

All depends on how many you'd like too!