Lawrence Watt-Evans 
Out of this World 

Pel Brown has troubles in his basement. But it’s not water leaking in – it’s magic. Sword-carrying barbarians are spilling through, demanding that Pel help them defeat Shadow, a dark force taking over their world. Meanwhile, on the other side of town, a spaceship has crashed in Amy Jewell’s backyard, and the aliens want Amy’s help against the Shadow seeking to conquer their world. When Pel and Amy go through the basement portal into the world of magic, the Shadow attacks and traps them inside. Now Pel and Amy find themselves entangled in escapades that will make them into heroes … or corpses.