An author who crops up again and again in my little green note book is Mary Stewart (1916-).
She wrote some of those gothic romance/mystery novels that I seem to have loved so much 30+ years ago. Several of her books were adapted for film and television.
The first of her non-Merlin novels (for which you may remember her) was MADAM WILL YOU TALK? published in 1955, and then again, rather adventurously, as part of an omnibus in 1956, which included NINE COACHES WAITING, and MY BROTHER MICHAEL. (I can hear you saying: I remember them!)
I'm obliged to Wikipedia for the plot summary for MADAM WILL YOU TALK?
Charity Selborne is on holiday in Provence with her friend, and former colleague, Louise. Before her marriage to Johnny Selborne they both taught at the same school in the West Midlands. Charity is now a widow, her husband's plane was shot down in France. She is staying at the same hotel as David Shelley and his step-mother Loraine Bristol. Mrs Bristol has taken David to France from England. David's father, Richard Byron, an antique dealer, who has been accused of murder, is pursuing his son across France.
Also staying at the hotel are John Marsden who is English and reads T. S. Eliot at breakfast and Paul Véry, who is French. Both have parts to play in subsequent events.Here's a list to look for, courtesy Fantastic Fiction - but you don't need to read them in order
Madam, Will You Talk? (1955)
Wildfire at Midnight (1956)
Thunder on the Right (1957)
Nine Coaches Waiting (1958)
My Brother Michael (1959)
The Ivy Tree (1961)
The Moon-Spinners (1962)
This Rough Magic (1964)
Airs Above the Ground (1965)
The Gabriel Hounds (1967)
The Wind Off The Small Isles (1968)
The Little Broomstick (1971)
Ludo and the Star Horse (1974)
Touch Not the Cat (1976)
A Walk in Wolf Wood (1980)
The Stormy Petrel (1991)
The Prince and the Pilgrim (1995)
Rose Cottage (1997)
Again, according to Fantastic Fiction, if you liked Mary Stewart, you'll probably like these authors
Victoria Holt (aka Jean Plaidy and Philippa Carr)
Phyllis A Whitney
Barbara Michaels (aka Elizabeth Peters)