Hexwood — Diana Wynne Jones

Strange things happen at Hexwood farm.

From her window, Ann Stavely watches person after person disappear through the farm's gate--and never come out again. Later, in the woods nearby. she meets a tormented sorcerer, who seems to have arisen from a centuries-long sleep. But Ann knobs she saw him enter the farm just that morning.. Meanwhile, time keeps shifting in the woods, where a small boy--or perhaps a teenager--has encountered a robot and a dragon. Long before the end of their adventure, the strangeness of Hexwood has spread from Earth right out to the center of the galaxy.