I just finished reading Son of a Witch [1], the sequel to Wicked: The Life and Times of the Wicked Witch of the West [2], and overall I found it lacking.
The book follows Liir, who may or may not be the son of Elphaba, the Wicked Witch of the West, from moments after Dorothy kills the Witch (quite by accident) for the next ten years. At the start, he's unsure of everything—unsure if he's the son of the Witch, unsure what to do, uncertain about what he can do. And after following him for ten years in the aftermath of Dorothy's visit and the poltical upheavel it brings upon Oz, he's still unsure if he's the son of the Witch, unsure what to do, and unsure of what he can do.
I think what made Wicked work was the recasting of the Wicked Witch of the West as the heroine; we get to see her side of the story in a politicized Oz. And while Son of a Witch carries on that story after Elphaba dies, it doesn't feel as tied into the Oz mythos as the first one. It's too far from any characters we know from Oz.
It just didn't work for me.