Secrets of the Ice Palace

This is the 17th novel in the Pitkirtly Mystery series.
Penelope Johnstone can't help feeling a bit sorry for herself as she spends a significant birthday alone at home. But while she is still wondering why her son Zak and his partner Harriet haven't visited her for a while, she gets involved in a sequence of bizarre events, ranging from being mysteriously threatened by the driver of an ice-cream van and receiving a delivery of six sacks of coal she can't use, to being stalked by a neighbour and helping a friend to deal with a man who claims to be smuggling maple syrup.
In order to take charge of the situation, Penelope teams up with former spy Amaryllis - always a high-risk strategy - and together they begin to uncover the secrets of the Ice Palace.