I'm not sure whether I would personally classify this as MG or YA. I think it's probably suitable for either level of readership. There's some hints of romance and a tiny bit of violence - at least high school kids getting into fights - but nothing that should take it out of the MG realm, and the vocab is probably suitable for MG readers. It's very easy to read. I flew threw it in one night.
The plot is completely unique. I read a lot of unusual, paranormal books for young folks and I haven't seen anything quite like this before. It hinges on the idea of a "thin space," a rare and unusual portal to the "beyond" where living humans can cross over (if they can find a portal) and speak to dead friends and relatives. The main character here is desperate to find a thin space so he can contact his twin brother who died in a car accident for which he blames himself. There are lots of cool twists and turns, reveals and reversals, particularly in the second half of the book. The characters are engaging and interesting and the main character has a lot to deal with emotionally.
I hadn't realized this was actually Casella's debut novel and I'm interested to see what she does next. I recommend this to younger and older readers alike.