The Northanger Abbey of schoolgirl books. Really fun.

I'm not sure if this was the best book to start with since it takes place in the middle of the series but a bit of confusion didn't take away from the general hilarity. Lloyd improves on Blyton by actually being funny instead of just describing that the girls are funny without any real humour. It makes for much more enjoyable reading and even the requisite moralising on how young girls should behave feels less didactic than you'd expect (although the relative progressiveness of their message doesn't make the writing itself feel any less shoehorned in).