"The Book of the King" is a middle grade/juvenile Christian fantasy novel. The authors would occasionally say things like, "...sulfurous breath--which means..." for words I think most middle schoolers would know, yet they also sometimes used words that some adults might not know with the meaning only implied by context.
The story was also told in an odd way. It's like you and the story teller are hovering above the scene and he points things out in present tense using "you" and "we" as if talking directly to the reader, then it'd slid back into past tense but it's like you're watching a movie. You never get inside the characters' heads, though you're sometimes told what they are thinking or feeling.
This gave the book a somewhat distant feel, but it still came across as an engaging story told by a (hovering) storyteller. There's plenty of fast-paced action and high suspense due to physical danger (though the distant feel made it so the really scary things weren't too scary). I'm not sure this story would really appeal to adults, but I do think kids of the right age would enjoy it. I liked its mood and subtle lessons better than I like a lot of Christian fantasy.
This book was the first in a series and it did end somewhat abruptly, but it's not left at a cliff-hanger. There was no bad language and no sex. Overall, I'd recommend this interesting novel to children.