Kingdom of the Cursed is the second book in the Kingdom of the Wicked series, andKingdom of the Wicked is the first part of this series. The author confirmed that it would be a trilogy.
While Kingdom of the Wicked was more a YA book, Kingdom of the Cursed is not. This is visible in the writing, but especially the amount of spice in the story. So I would say that Kingdom of the Cursed is more a NA book.