Under the Light is actually a companion novel to Laura Whitcomb's book A Certain Slant of Light. It is totally not necessary for you to have read it in order to read this one. While yes this one literally does pick up within minutes after the other book ends - this one is instead focused on two young teens named Jenny and Billy. 
 Finding each other 
 Jenny has a really rough home life. Her parents are so religious and her father so strict about everything that one day after throwing away so many of Jenny's precious personal belongings Jenny snaps and her spirit leaves her body and she floats about the world experiencing everything she can while leaving her body as what she believes to be an empty shell. But unbeknownst to her Helen (the ghost from A Certain Slant of Light) occupies her body for a very short time and uses her body for her own ends with another ghost, James who has occupied another empty body (Billy's body). 
 Billy has it just as rough as Jenny does. While he doesn't have fanatically religious parents he has absentee ones. His father is in jail for beating his mother into a vegetative state and his mother is in the hospital. He's fallen in with the wrong crowd and to try to escape his life he turns to drugs and somehow ends up detaching his spirit from his body. While he's away he's occupied by the ghost James. 
 Now the period of "occupation" by the ghosts is what is covered in A Certain Slant of Light and while it does have an effect on Jenny and Billy when they return to their bodies, as I said you don't need to have read that book to be able to follow along with this one. It was definitely enjoyable and more of a crossover adult read whereas this one is a young adult read. So if it sounds appealing I would recommend it. 
 While Jenny and Billy are aware from their bodies they happen to meet in spirit and become close but when they are thrust back into their bodies neither one of them can remember exactly the time they spent out of their bodies. But they do have evidence that the two of them have had a relationship that they both can't remember. Slowly they start to piece this together while navigating their tumultuous lives and problems. 
 I have to admit emotions ran high for me when it came to Jenny's side of the story. The book is told completely from her and Helen's perspective as Helen has decided to stay on earth to try and protect Jenny from any harm she may have caused while she had occupied her body. Jenny has it really rough as her parents are now getting a divorce and her mother is not being a very good supportive parent (not that she ever was to begin with). I also have a very hard time whenever fanatical religion comes into play when I'm reading. It's a subject that makes me uncomfortable to read and I know this directly effect my enjoyment level of the book. But I think this was supposed to be an emotional book and it definitely succeeded at wringing the emotions from me. There was a lot of self discovery and a bit of coming of age for both Jenny and Billy (though you really learn more about Jenny since we don't get Billy's perspective). They are both so young and have gone through so much and seeing how those circumstances shape them and yet they are still good people and the choices they make is a really fabulous thing to see. 
 While it was a sad read it also comes with a message of hope and I believe imparts a message to teens that they do still have power over their lives. It's definitely a book I would recommend to any young teen.