Unless you’re a librarian, you probably didn’t see the recent article by Don Crinklaw, “How Charles Todd Met Hamish McLeod: The Mother-Son Team Behind Inspector Ian Rutledge”. https://bit.ly/2sle9gT
The article talks about the authors, Charles and Caroline Todd, as collaborators. It also talks about Hamish McLeod and his relationship to Ian Rutledge. The two made their first appearance in 1996 in A Test of Wills.
Don’t hesitate to look for Charles Todd’s mysteries in the Web Store. https://bit.ly/2rf5pFH