Note: This is a review of the pre-release edition of The Oz Engima. This version was available in paperback at the Chesterton, Indiana Wizard of Oz Festival, and the final edition of the book may be revised or altered to some extent.
In the story, Dorothy and her friends are joined by new characters like Mr. Yumkin on a journey into space to ultimately stop a witch named Maelstorm and her allies Blackheart, the newly-resurrected Witch of the East, and the Nome King from destroying the Emerald City.
I think that it's a brilliant idea, but pretty poor execution. The villains in this book is definitely where some of the biggest issues with the story lie. For instance, we are told near the beginning of the book that the Wicked Witch of the East has somehow been resurrected (or perhaps she never died). No further explanation or development there. One of his more recent books The Oz Odyssey shared this same redundant plot device.
Another villain in the story is Maelstorm, who is the cousin of the Witches of the East and West. She's basically just an over-the-top, stereotypical witch. She even says something at one point along the lines of "I love being bad because I'm a witch!". I think it would have been better to just ditch the wicked witches all together, and instead have an original, new villain who isn't related to the original Wicked Witches.
Something that is evident in this book and in The Oz Odyssey is that it doesn't seem like Roger has even read any of his great-grandfather's books. And if he has, his stories do not fit anywhere in the timeline of the original books. Yes, you could say that Roger's stories take place in the present-day Oz, but that's not even possible as somehow the same Nome King is still evil and plotting to take over Oz with no explanation as to how he's turned evil again.
There are also strange and unexplained references to the MGM movie, such as farmhands back in Kansas named "Huck and Hank", mentions of the Wizard of Oz giving the Cowardly Lion a medallion of courage and the Scarecrow a diploma, and more that I can't recall off the top of my head. There are even inconsistencies in Roger's original material. For instance, there is a land that is sometimes referred to as Candy Country, Candy County, or Candy Land. Which is it?
I think it's obvious that this version of the book was either rushed or possibly just poorly proofread and edited, because there a ton of errors throughout the book, and really hurt the overall quality of the story. I'm not sure how different the final edition of The Oz Enigma will be, but I unfortunately have to say that I would not recommend this to an Oz fan in its current state.