The Interior People by Thomas Heffernan

The Interior people
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'There are gods here on earth, Magnus. But not all of them know they are gods. Some are lucky enough to be born knowing what they are. Others have to figure it out for themselves along the way.'

Magnus, a hired killer desperate to atone for the things he has done, is visited by the ghost of his first hit, a mysterious man named Byrd.
Byrd offers Magnus redemption, a journey into the lives of the people left behind after he killed their loved ones. Byrd tells him it is not too late to save these lost souls.
One of these souls is Sam, slowly destroying herself after the murder of her fiancé.
As Magnus and Sam drift closer together, the world around them begins to fall apart, and Magnus realises the one soul he wishes most to save may just end up saving him.