Singularity — William Sleator

It started as a two-week vacation, and turned into a terrifying journey out of time.

Barry and Harry Krasner are identical twins, but that's where the resemblance stops. Barry's more athletic, more aggressive -- and he's the one who suggests that they house-sit their great-uncle's deserted farm. Harry hopes that it will bring the two of them closer. And it does -- because there's something chilling about the farmhouse, something that makes the locals stay far away. The twins are sure that the locked shed on the property is the reason why -- but what they find inside is far more horrible than their worst nightmare.