I've read The Children of Green Knowe, The Chimneys of Green Knowe, and The Stones of Green Knowe. All are excellent children's books appropriate for mid- to late-elementary ages. I particularly enjoyed the last of these because there is no TV or movie version of it, so the story was totally new to me.
The three books are similar in that they all involve children who belong to one very long family line (from 1120 to about 1950), the Oldknowe family. Some critics say these books are about ghosts, but that's not an accurate statement. The point regarding the children is not that they are dead. The point is that they are perpetually alive and able to drift in and out of each other's "present." So, these are not children's stories full of scary ghosts. Far from it -- they are stories full of children who long to know their distant relatives and are given a magical chance to do so. I found the idea enchanting and wonderfully effective in her plot.
The other three Green Knowe books do not have this magical theme as much. I'm now reading An Enemy at Green Knowe, which apparently has a little of this theme, but the other two books (The River at Green Knowe and A Stranger at Green Knowe) lack it entirely. In addition, those books do not have the same children in them, particularly the 20th century boy named Tolly who figures in the other books.
I've enjoyed reading about Lucy Boston, her actual house (The Manor at Hemingford Grey in Cambridgeshire, England), hunting pictures of the house online, and reading an interview with her daughter-in-law Diana who now runs the house. Lucy was evidently a fascinating and independent woman.
Lucy Boston and I have this in common: we both love children's tales with magic and adventure, and we both love old houses that contain those children, that magic, and those adventures. Apparently she did not begin writing about Green Knowe until she was at least 60, so I'm not feeling the panicky time-crunch so much these days!