This book is contemporary with fantasy elements (you might call it magical realism, but more horror than charming), and centers around 13 year old Conor, but I wouldn't call it Young Adult. It's got a very human theme at the center, and it's not written for kids but for anyone who has undergone loss of a loved on and come to understand grief.
Herne is perfect, Conor is absolutely perfect and believable, the grandma is absolutely believable....Patrick Ness has a gift for character development, tight stories, and emotion. Apparently, Siobhan Dowd started this but died before finishing it, and I think Ness does a beautiful job of bringing the story to life with respect.
So, of course, I recommend it for anyone who has lost a loved one (especially if it was prolonged by illness or accident), or may be going through terminal illness themselves. It will make you cry, but it's a healing kind of cry. And absolutely worth it.