Series numeration from goodreads.com

A young woman, Fiona Holland, has gone missing from a small Irish village. A search is mounted, but there are whispers. Fiona had a wild reputation. Was she abducted, or has she run away? A week later, a gruesome discovery is made in the woods at Ireland's most scenic beauty spot - the valley of Glendalough. The bodies are all young women who disappeared in recent years. D.I. Tom Reynolds and his team are faced with the toughest case of their careers - a serial killer, who hunts vulnerable women, and holds his victims captive before he ends their lives. Soon the race is on to find Fiona Holland before it's too late.