There are currently four novels in the Ethereal World: a duology, The First Ethereal (Book 1) & The Blessing of Crows (Book 2) plus two standalone prequels The Magic Keepers & The Butterfly Witch. 

Reading order:

If you’d like to read them in the order they were written, start with the duology then read The Magic Keepers and The Butterfly Witch last.

If you like reading in historical order, then it’s The Butterfly Witch, The Magic Keepers and then the duology.

The books are all connected and easy to follow so where to begin is up to you.