The Collector by Gillian St. Kevern

No one escapes the Collector.

All Gideon Lawes has left is his scrupulous honesty. Employed to investigate a supposedly haunted house, Gideon vows to uncover the secrets of 32 Belcairn Road. But he gets more than he bargains for in the form of the Collector, a spirit relentlessly pursuing an unpaid debt.
Drawn by chance into the lives of cheerfully generous Fairweather and darkly ironic Holford, Gideon discovers things about himself he never imagined. But with the Collector closing in, Gideon must choose between destroying the friendship he values most or sacrificing his self-respect for a lie. Whatever Gideon chooses, the Collector will claim another victim.