Philippa, you asked about the Ellis Peters detective novels !
They are absolutely wonderful, set in the 1950`s in the Welsh Marches on the whole.
I would start with "A Nice Derangement of Epitaphs" or "City of Gold and Shadows".
"Holiday with Violence" is about a group of teenagers who get mixed up in a robbery in Italy. This is one of my personal favourites.
"Black is the Colour of my True Love`s Heart" has a music theme, as does the "Funeral of Figaro"
"The Knocker on Death`s Door" is also a brilliant read.
All of her detective books are gripping, and I can pretty much guarantee you won`t be disappointed :-)