The Phantom Eye is the first in a series of detective fiction novels by JP Jackson.

High in the Scottish hills in the grand estate of Black Manor, a hit man dangles from a chandelier, his neck broken. 
The recently deceased owner of the manor, Sir Montgomery Wood, is said to have been in possession of a diamond necklace once worn by Cleopatra. 
With a killer on the rampage and the worldΓÇÖs criminal elite in a race to find the priceless diamond, MontgomeryΓÇÖs son, Cary Wood - a reclusive schizophrenic with a talent for detection - takes on his first case.

A good yarn, short, punchy, dark and funny. Definately worth a download.