The Girl Next Door by A.J. Rivers

Sherwood is a sleepy little town. A town full of friendly faces, and even friendlier neighbors.
Which is why FBI agent Emma Griffin isn't surprised when her new neighbor starts stopping by to chat.

Turns out her neighbor has a dark past and secrets that made her run.
As much as Emma wished it wasn’t true, dark past and secrets are things that she is far too familiar with, especially now more than ever.
A chilling unknown filled the air of Sherwood, Virginia.
Strange things are happening around Emma.
Things that no one can explain, and many don't believe.
And when Emma witnesses two figures hidden by sheer curtains, strangling a smaller figure.
A figure she can only assume to be Ruby's.
She becomes a witness to the murder of the girl next door.
But what do you do when no one believes that the girl even existed?
Emma’s world starts to spin, and she begins to wonder…
Is someone taunting her and taking lives to torment her? Or has she truly gone crazy?

The Girl Next Door is the fourth book in the Emma Griffin Mystery series, it can be read as a standalone.