I had to look this one up a few times because honestly, 'The Inbetween' isn't the most unique title—there's a few books floating around with that name. I think you're talking about the one by Katie Hayoz, which is a YA paranormal romance. The main plot follows this girl, Tessa, who has a kind of ghost-touch ability, where she can see and interact with spirits stuck between worlds.
It's really a story about her trying to solve the murder of a ghost named Finn while dealing with her own grief over her father's death. So it's a mystery, but wrapped up in her personal struggles. The 'inbetween' is literally the place these ghosts are stuck, and Tessa's sort of stuck there emotionally too.
I remember the dynamic with her mom felt pretty strained and real, which added a layer beyond just the spooky stuff. The ending ties the murder mystery and her family issues together, though I felt the romance with a living guy was maybe the weakest part for me.