Starve Acre by Andrew Michael Hurley

The worst thing possible has happened. Richard and Juliette Willoughby's son, Ewan, has died suddenly at the age of five.

Convinced that the boy still lives on in some form, and desperate to make contact, Juliette seeks the help of the Beacons, a seemingly benevolent group of occultists.

Whereas Richard, an art historian, tries to blot out the pain of his grief by turning his attention to the field opposite their house, Starve Acre.

Patiently he digs in the barren soil looking for the roots of a legendary oak tree but unearths something which ought to have remained buried.