Unnatural Issue is the story of Suzanne, who has grown up in her household without the acknowledgement of her father. Obsessed with his grief at the death of her mother, he blames his daughter for his wife's death, and the staff has orders that he never set eyes on her.
As she comes of age, she falls into the care of The Puck (of Shakespearean fame), who, in Lackey's world, is a nature god who teaches young Suzanne all she needs to know about caring for the elemental earth. One day her father sees her--for the first time in years--and conceives a plan to use forbidden necromantic powers to bring his long-dead wife back to life.
Loosely based on the Donkeyskin story-type, and set against a background of burgeoning WWI, Lackey's story is a fairly quick, interesting read, a bit bogged down by her tendency to tell and not show, and her characters' needs to discuss, mentally or aloud, every aspect of their decisions. In spite of that, it's an engaging story, and the protagonist is certainly sympathetic. Lackey throws a hitch in her usual romantic story by having Suzanne besotted with a man who shows no return interest, even as the other main character falls for her. The wrap-up to that part of the storyline felt a bit sudden and unsatisfactory to me, but all is well and so--happily ever after.
I did like the way Donkeyskin was incorporated. It was not as straightforward as I would have thought, and is mostly in the setup of the story, which Lackey takes from there to make her own.
If you've enjoyed the other entries in this series, I recommend this one. A few characters from the other book make minor appearances (although it's been long enough since I read the others that I only had vague recollections of the stories they were in). It's about on par with the others in the series.