Mary Stewart: Madam, Will You Talk? [1990] paperback

Charity had been greatly looking forward to her driving holiday through France with her old friend Louise - long, leisurely days under the hot sun, enjoying the beauty of the parched Provencal landscape. But very soon her dreams turn into a nightmare, as Charity becomes enmeshed in the schemes of a gang of murderers, one of them a man with whom she is rapidly falling in love ...