A Certain Slant of Light by Laura Whitcomb.
Whitcomb begins with an intriguing premise. What if, after you died, your spirit continues to linger among those living on earth for years to come? What if one of the ones called Light found a recently abandoned body and decided to inhabit it?
After a boy overdoses on drugs, James enters Billy's body. Though James is far older than a teenager, he becomes "Billy" to all those around him. As Billy, James can "see" Helen in his high school classroom, even though no one else can.
Helen is a first flustered and then delighted to be able to communicate with someone. For 130 years, she has simply lived as Light, haunting people she calls Hosts. Her last host was Billy's teacher, Mr. Brown, but everything changes after Billy/James can see her.
Helen and James become romantically involved, with James urging Helen to borrow a body, too. Though she has qualms about it, she borrows the body of a teenage girl. With their borrowed bodies each of them begins to have memories of their previous lives. James had been a soldier and Helen had been a mother of a two-year-old.
Helen and James both know they cannot keep the bodies they inhabit, so they devise a way to help the departed return.
This a young adult novel with a lot to offer to adults and teenagers. Characters, especially James, are well-developed and believable. The premise, though far-fetched, is one that intrigues and, in the end, delights.