The Missing: Crocodile Spirit Dreaming Books 4 & 5

It is a search for five vanished girls, backpackers last seen in outback Australia. Fragments and clues are found but no solid traces. A man's dairy tells their story.
Finally one person emerges from obscurity without memory who may be one. and gradually the pieces of her disappearance and that of the others are reassembled.
These two books in a box set tell the final part of this series story.