Oooh, I love Anne of Green Gables. Honestly, I'd give the original, unabridged version a try. I re-read it recently, and although it has some old fashioned words, it's not horribly complicated. I don't think it's more grownup than the Little House series, really.
I don't know why, but the doctored down versions of the books (like splitting Little Women into 2 shorter, abridged, books, etc) just doesn't feel right to me.