The Legacy — Alan Grainger

When a solicitor who specialises in finding lost heirs turns up looking for Charlie Cassidy, a professional writer of fiction, and finds he'd died four years earlier, he tells Max and Millie, Cassidy's son and daughter, he has information which might connect them, through their late father, to an unclaimed legacy. Millie's husband, who is a solicitor, thinks they should reject the heir hunter's expensive offer, saying he'll process their claim if they trawl through their father's things and come across anything pointing to an obscure family connection of which they had no previous knowledge. The hunt upon which they embark uncovers many of their late father's secrets and leads them to conclude he wasn't the man they'd believed him to have been. This is the third book in the Templederry Trilogy. The first is The Learning Curves, the second is Father Unknown