"Forgotten Girl" is the first book in a series (which does explain the cliff-hanger at the end). The "Steampunk OZ" series is positioned for mid-grade (8- to 12-year-old) readers ... but I would say that it really depends on the reader and parents may want to prepare themselves for some discussions. A lot of the material is pretty dark, as the tale begins with Dorothy Gale losing her parents and going to live with Uncle Henry and Aunt Em in New Kansas.
Because it's steampunk, there are airships and magical devices all over the place, but not in a "we slapped these in here for no reason" way. The mechanical wonders advance the plot and make for some scientific interest at the same time.
This is an entertaining start to a series, and I look forward to reading more in the future.