Gumtree, the Winner! yes, it's Joanna Trollope
She was born in 1943 and lives in Britain; she has written about a dozen novels, and almost that many historical novels as Carolyn Harvey.
Her distant, famous relative is Anthony Trollope, whose novel count was about 55, not including short stories, plays, and travel books. I especially like his Autobiography.
Let's just reveal the book and get on with the game:
The Best of Friends: the young protagonist is named Sophie.
I'd call it beach reading, and was intending to get in the mood for summer, which has hardly arrived here at Cape Cod, where it's been record-breaking cool and rainy all month.