** spoiler alert ** 
 Read this at the recommendation of the author of my longtime favorite Labyrinth fanfic called The Goblin Market (inspired by Christina Rossetti's poem of the same name and, of course, this book, The Hollow Kingdom). I'm going to assume that THK was inspired by Christina Rossetti and the Labyrinth, also. 
 I very incorrectly assumed that this book would also be adult in theme due to the recommendation coming from a very adult fanfiction. Folks, The Hollow Kingdom is SO G-rated and SO maddeningly innocent, which is a shame because Kate and Marak had a wonderfully sarcastic repartee in the first half of the book that would've made for some interesting scenes later. 
 My main gripe with the story is the jump in time once Kate arrives to the goblin kingdom. After going through the marriage ceremony on her first day underground, there is a 1.5 year leap forward, and we know nothing about what happened to each character in that time, and almost nothing about the romantic developments of Marak and Kate. Though, at the end Kate finds herself pregnant with an heir. Also at the end there is a mini-battle with a sorcerer, and a sad, unnecessary slaughtering of caged animals. Then Kate has her baby and everyone lived HEA. 
 tl;dr First half of book is adventure packed and GREAT because, hello, chase scenes. Last half felt rushed because we don't really know what the characters have been up to, and then there's a murderous sorcerer who comes out of nowhere.