A Traveller in Time, by Alison Uttley, is a classic timeslip story. It's an early one-1939, but has been reissued several times, most recently in 2007 by Jane Nissen Books, whose mission is to bring classic children's books back into print (hooray for that! and for the fact that the dollar is almost fifty cents stronger against the pound than it has been).
In early 20th century England, two sisters and a brother are sent from their London home to stay with relatives at a farm in far-off Derbyshire. The youngest, Penelope, has always had a touch of second-sight, and here, in the ancient manor house where her family has lived for centuries, she begins to catch glimpses of people from the past. And then she herself travels back to the age of Queen Elizabeth. There is her aunt, working in the kitchen, who accepts her as a niece newly come from London. In what is one of the easiest time travel experiences I've ever encountered, Penelope is able to fit in, with little questioning, into the Elizabethan household, moving gently back and forth between past and present with no awkward outside mechanism.
It is a household under strain. The Babbingtons, whose manor it is, are Catholic supporters of Mary, Queen of Scotts, imprisoned nearby. Anthony, the head of the family, is her fervent supporter, scheming to free her. Penelope becomes caught up in the Babbington's partisanship, and although she knows that Mary's story does not end well, each time she travels back into the past the present seems cloudier...until she is almost more at home back then, befriending Anthony's younger brother, affectionately looked out for by her "aunt," and becoming more involved in the Babbingtons' plot to free the queen.
"Somehow, in the serenity of the sunny, bare chamber where we stood, with Master Anthony's blue eyes looking into mine, and that lovely jewel dangling in his fingers touching mine as he spoke, I felt all things were possible. We would put back the clock of time and save her.
He took my hands in his, and held them tightly.
"You want to save her? I can trust you with my heart? You love her too, Penelope?" he whispered.
"Oh yes, Master Anthony," I cried passionately. "I love her too," and my words were true." (page 108 of the Puffin version, the ugliest cover of the three I found, and the one I own).
Alison Uttley is a slow and generous story teller. Her descriptions are thick, and her attention to physical detail acute. Both the Elizabethan manor and the early twentieth-century farm come to vivid life. The story is told in the first person, allowing Penelope's feelings to come through clearly, and she is a likable, trustworthy narrator, but she seemed to me to exist more to be an observer of the Babbingtons than her own person. I think (supported by various reviews on Amazon, and comments of acquaintances) that if this is one you read at a child, at that impressionable age when a story can easily become a Story, it is one that will be loved. As a grown-up, it felt a bit slow, a bit loose around the edges of its time-travelling. I would, quite frankly, have liked to have seen more of the "three children at an old farm" story-line, although I realize that Uttley was not writing that book, and so it is not a valid criticism. But still, this is a book well worth reading.
Not least for its educational aspects (and for those of us who learn history best through fiction, this is important). Anthony Babbington was a real person, who really did give his heart to Mary, Queen of Scots, at the cost of his own life. His home, so lovingly described by Uttley, still stands, and is now a bed- and-breakfast: