If I remember correctly, most of the mysteries in this series do not start out as normal police investigations. Often they originate during or from some event in the Tibbett's lives. They may be on vacation and a murder occurs, that type of thing. That is not always true, however, as in Murder a la Mode (covered here and here by Moira at Clothes in Books).
This book is one of my favorite of the series because of the the focus on Britain during World War II and how the war affected peoples lives. The book was published in 1965 and looks back at a time twenty years earlier.
The review of Johnny Under Ground at In Reference to Murder gives some of Patricia Moyes' background, including her work during the war as a flight officer in the WAAF. Obviously Moyes used her experiences in writing this book. At Rue Morgue Press, there is an article with even more information about her life, written by Katherine Hall Page.
Publisher: Henry Holt and Co., 1987. Orig. pub. 1965.
Length: 253 pages
Series: Inspector Henry Tibbett, #6
Source: I purchased my copy.