Beyond Silence, by Eleanor Cameron (1980).
But then. Such a sad thing to find that this is actually a pretty bad book.
Oh Eleanor Cameron. What were you thinking? I love Court of the Stone Children, I re-read A Room Made of Windows and the other Julia books lots....you were good, Eleanor, at bringing to life smart, introspective girls. So why did you think it would be good to write about a teenage boy? It didn't work.
Andrew, the young protagonist, has come to Scotland to stay in the ancestral home, now a hotel. His brother died after coming back broken from the Vietnam War. His parents' marriage is on the rocks. And Andrew himself is battling demons in the form of haunting nightmares about his brother's death, in a car accident whose immediate aftermath he saw. And now in Scotland he is being haunted, but much more magically and pleasantly, by a young woman who lived there a hundred years (or so) earlier--Deirdre, who once was loved by another young Andrew, and who knew tragedy of her own.
The timeslip bits involving Andrew seeing Deirdre in the past, and occasionally her hearing/sensing him, are rather pleasant and not fraught with tragedy--it's time slip as window between times. But the Point of it all, the connection I assume was happening in Eleanor Cameron's mind between the time travel and Andrew's mental healing went right over me. Sure it gave Andrew something to think about, and put him in a state of open-ness that let him recover the memories of the details of his brother's death, but that isn't all that much of a connection. And sure it's always nice to read about people time slipping around in a pretty uneventful way, but the level of interaction between times was never great enough to be a worthwhile story arc in its own right.
Instead, what we get is Andrew (boringly) rehashing the same things over and over again in really over-wrought prose:
"The rain was swept and driven all night long. Even in my sleep, I was aware of it, and that I struggled, not physically, but in my mind, and this struggle was so exhausting that I could have cried for mercy, yet I would not let my struggles go." (page 131)
Turgid prose. I was all, like, get over your clauses, Eleanor. Even as I read, I struggled, not physically, but mentally, as the book continued on its melancholy way, and yet I had to finish it.
And then when Andrew achieves peace, he ends up with Deirdre's portrait which he hangs in his college dorm some years later. Which is odd, and probably not going help him socially.
Also odd is Eleanor Cameron trying to write about teenage male sexuality in a convincing way, and not being wildly successful.
Short answer: don't bother unless you are a romantic introspective young teen reader from 1980, back when satisfying books were thin on the ground. The time travel isn't enough to make it worthwhile to read on that account, and the rest of the story doesn't make up for it.