The Last Post — Gillian Larkin

A short mystery with ghosts.

Grace Abrahams helps ghosts who can't move on until their problems have been solved. The ghosts she meets are attached to items found at storage locker auctions.

The day that Grace feared finally arrives; the day when she meets a ghost she knows.
The ghost is Roz, a postwoman She had been delivering post to Grace's shop for years, and even knew Grace's late parents.
Once Roz gets over the shock of being dead, she tells Grace what her unfinished business is. Roz delivered post to an old lady called Olive, and Olive is in trouble and needs urgent help.
Grace doesn't hesitate to help and is soon deep into her investigation. Things take an unexpected twist when an attempted murder is revealed. The would-be murderer won't rest until they claim their victim.