I think the important thing for the books is that they can stand on their own, even without the "OZ" source concept. To me, it was just a lot of fun to use the overall concept and the characters, but only to a point and as more of a starting place.
In this story I'm charting completely new ground. The character types are there (a metal man, a coward, a brainless traveller) but they are so different from Baum's characters and have so much of their own personalities that they truly do stand apart from The Wizard of Oz.
The struggles that Dorothy faces are also unique to this series, taking her in directions that even I hadn't planned. That's the truly exciting thing about creating characters with a life of their own: they say things and do things as you write them that you hadn't planned in advance!
So, yes, this book series is inspired by The Wizard of Oz, but it's nothing like the Pride and Prejudice and Zombies type of book. It really is it's own unique story. I hope you enjoy reading it!