When Marnie Was There, by Joan C. Robinson (1967) is a lovely example of the sort of character driven, atmospheric, haunting story that's my favorite type of time travel.
Anna is an orphan, with foster parents who love her, but who have never been able to make her feel loved. Withdrawn to an alarming degree, with no friends (she's not really even fond of herself) she holds herself tight within a shell of indifference. When she is sent from London for an extended stay on the coast of Norfolk, to build up her strength, she spends her days idly exploring the shore, drawn in particular to an empty house by the water....a house that feels strangely familiar. A house that isn't empty, after all.
Because Marnie lives there--the kindred spirit who Anna had never dreamt of meeting. Marnie, whose parents are rich and fond of her, but who, like Anna, is lonely and neglected. Marnie, who appears almost out of no-where, and who fills Anna's thoughts...
And when Marnie must go back to the city, she has left Anna the gift of being a person who can have friends... and Anna finds herself drawn into the large family who have moved into the old house, filling its emptiness with love and warmth. Her memories of Marnie fade like a dream, until the revelation comes that Marnie and Anna are connected more deeply than either girl could have imagined.
Marnie wasn't actually there after all; somehow Anna had gone back into her time (just before WW I). But it isn't a story about an adventure in the past--Anna doesn't even realize that Marnie was from an earlier time. Instead, it is a story about friendship, and how Anna's character changes as a result. So don't pick this one up if you want Excitement. Do pick it up if you want gripping and poignant introspection. Highly recommended to my ten-year old self, and to any girl who feels out of step and alone.
My only quibble is with the overly tidy and hasty wrapping up at the end...surely Anna's foster parents had made enquires about her family? Did she really have no living relatives?
Harper Collins republished it as a "modern classic" in 2002, so although at the moment it's a tad on the expensive side in the places I just looked, if you keep you eye on it you can find a reasonably priced copy (I paid about $5 for the one I just bought--thanks, blog reader Julia, for recommending it to me!).