The Cuckoo

The Cuckoo marks the welcome return to the coastal town of Fjällbacka, and two of its residents, Patrik and Erica.
Patrik finds himself investigating the murders of Rolf, a photographer, and those of Peter and his young sons Max and William.
Erica meanwhile, is looking into the unsolved murder of a trans woman and her young daughter in Stockholm in 1980, with a view to writing a book about it. It soon becomes clear that there is a link between Patrik's cases and the cold case.
The story is slow to begin with, but as soon as the murders begin, the pace quickens.
There are lots of plot twists, but despite these I had figured out whodunnit before the big reveal.
A dark, gripping read.