There are two things you need to know about Miss Silver, private detective: She knits while she solves crime, and she speaks in Cough.
I always noticed that she coughed a lot, but in this book I was really impressed by the variety of things she's supposedly getting across to her audience. In the space of two pages I found coughs that were 'introductory', 'gentle', 'gracious', and 'hortatory'. I've decided to accept the absurdity, which may have been easier this time since our intrepid detective hardly features in the book at all.
All that aside, this may be my favorite Miss Silver to date. Patricia Wentworth can be quite witty and observant when she wants to be, and the whodunit is textbook. House party in the country, everyone has a motive, etc. etc. Sometimes that is exactly what you want to read.
Age I'd let Z read it: 11 or 12.