The Sherwood Ring, by Elizabeth Marie Pope (1958), is one I wavered about ever putting in the "time slip" category, because "ghost story" fits it just as well. But the particular ghosts in this story aren't exactly haunting the old house in upstate New York, in apparitionly way. They are more like family members, dropping in from the past to visit with newcomers, with whom they converse like ordinary people, and, like good time travellers, they aren't their dead selves (ie, they appear as their younger selves). And since they are in fact family members, with strong attachments to the place, and some natural interest in their kinfolk, why not think of them as sliping forward in time? So I did.
The kinfolk in question is newly orphaned Peggy Grahame, come to live with her uncle Enos, whose a recluse obsessed with the family history. So territorial is he, intellectually, that he forbids Pat, a young British scholar (also interested in the doings of the Grahame family in years gone by) to ever darken his doors. Since Peggy and Pat had become friends on their journey to New York, and since there is absolutely no one else for Peggy to be friends with, this is a blow to her.
Fortunately, there are other someones--a cast of characters from the Revolutionary War era, who visit Peggy and tell her their stories.
Now, when I realized this is what was happening, I was very doubtful. The stories that Peggy is told are separate narratives, and at first I thought I was going to be presented with a pastiche of "stories from history," thinly tied to Peggy's own story (which I was very interested in--orphan, old house, romance, all that). I didn't want to be taken away from it.
But then, when I realized that the visitors were all actors in a very romantic, very exciting drama, and when one of them in particular started reminding me very much of the Scarlet Pimpernel, I was hooked by their interconnected story of torn loyalties, espionage, daring deeds, and other Revolutionary War reindeer games. In short, it turned out to be a great read--really fun historical fiction, with romance of that nice grin-making kind (as opposed to introspective-angsty type of romance, if you know what I mean?) in both past and present.
Note on age--I would have loved this when I was nine or so, and enjoyed it last month. So there you go. I am not surprised, though, that I didn't read it when I was young; it has had bad luck with covers (two others shown below), and American History didn't appeal to snooty little Anglophile me. My loss.