Ravine, by Janet Hickman* (2002, Greenwillow, 215 pp). In the Dark Ages in northern Europe, Ulf toils for an evil queen, a foreign slave without hope in a harsh world. In present day America, Jeremy and his best friend Quinn play with their model warriors, still enjoying childhood. Jeremy's mother is engrossed in her work--translating an early medieval tale, and has little time to keep an eye on him, warning him to stay out of trouble.
But trouble comes. Jeremy's collie, Duchess, disappears down a forbidden ravine...and wanders into Ulf's time. Ulf is delighted to have the companionship, however brief, of the beautiful dog, but his need of her widens the path between past and future. Soon Jeremy and Quinn, with Quinn's unwilling big sister following, climb down the ravine themselves, looking for Duchess. In the dark and violent world where Ulf lives, where he is living through the very tale that Jeremy's mother is retelling, survival is a sometime thing, and getting home might prove more dangerous, and heartbreaking, than Jeremy had ever imagined.
This is a rather charming adventure story. It's not tremendously complicated, it's not a very good history lesson (I found it a bit vexing that I wasn't sure where in Europe I was, and if everyone was Germanic, or possibly some were Viking), and the magic isn't explored to any sort of conclusion. Yet the characters are real and worthy of the reader's care, and the story moves along an exciting and engrossing path. I wouldn't rush to recommend this one to adults, but I will be giving it to my boys in a year or two.
*best known for her 1994 book, Jericho.