Friends in Time, by Grace Chatwin (Macmillan, 1992, middle grade, 127pp).
Emma is facing yet another move--the loss of her best friend, and the lonely prospect of a new school full of strangers. Visiting the old abandoned Bently mansion next door for the last time, Emma wishes for a best friend forever, and, as if in answer to her prays, Abigail Bently appears. She's been whisked from 1846 to Emma's time by the power of a shaman's doll, appropriated by her merchant uncle on his last trip to Brazil.
Emma is thrilled to have a new friend, and delights in showing off such wonders as modern plumbing and electricity. Abigail wants nothing more than to use the doll to go home again...only Emma has hidden it, hoping to keep Abigail as a friend forever.
"Abigail will be much happier here and now, with me, she told herself. Emma thought again of all the fun she'd have, showing Abigail the present day. Going on bike rides together...
Tonight, the bathroom plumbing: tomorrow- the world!" (page 67)
But when at last the way through time opens again, Emma, to her horror, goes back with Abigail. There she must hope her "friend" forgives her enough for trying to keep her as a pet to send her home again.
This is, at its heart, a story of friendship, for which the timeslip serves as a setting and gives added interest, and it's rather successful in its portrayal of the difficult relationship between the girls and the hunger for a best friend that lonely girls often feel. But because this element of the story is more important than their time travelling adventures, the reactions of each girl to the other's time are somewhat superficial, and there's only a small touch, here and there, of the creepy disorientation that often comes with the genre. The doll from Brazil, the mechanism that causes the timeslip, is a somewhat disturbing device--it never quite goes beyond the neo-colonialist cliche of the Dark Magic of Native Peoples, so much so that I don't particularly want to recommend this book--but, to do it justice, it works rather well to make the story cohesive, providing both impetus and explanation.
In a nutshell, this isn't one I'd particularly recommend as a timelip qua timeslip, and I have issues with the doll, but in all fairness, it's a perfectly readable story of middle school friendship with the time travel element making it memorable.