You gotta love a good prequel!  And this is a good one!  It was so smart to concentrate on these two characters, they were a large part of what I loved from Every Heart A Doorway.  I am completely satisfied with this story arc, I wanted to know what turned Jack and Jill into what they are.  It was brilliant to include the bits about their parents, how Jack and Jill were raised.  Rebelling against the stereotypes they were forced into, warping their personalities so they accepted the world they entered into with little fuss. For such a slim novel there was a lot to digest.  I think you could read this book first, if chronology matters a lot to you, I may read #1 again with this better understanding of the twins, but I think Every Heart A Doorway sets the reader up to want more of a taste of the backstories of the characters.  I am fervently hoping to hear that McGuire is going to follow up with more prequels, one for every child, one for every adult in the Wayward Children universe!