Really enjoying this thread! I loved Marianne Dreams, Tom's Midnight Garden, The Green Knowe books, Charlotte Sometimes, The Ghosts (Amazing Mr Blunden), The Chinese Egg etc.
I also loved Fifteen by Beverly Cleary, which I don't think anyone has mentioned. It's about a fifteen-year-old girl's first experience of dating in the US in the 1950s. I borrowed it from the library as a teenager in the 1970s, and loved it. I bought a copy a couple of years ago, and it still takes me back to what it was like being a teenager! I also enjoyed her other books - The Luckiest Girl, Jean and Johnny and Sister of the Bride.