The Night the Heads Came — William Sleator

Leo and Tim are abducted by aliens -- and find themselves in the middle of an intergalactic feud

Leo is driving his best friend, Tim, to the station to catch the midnight train to New York City, where Tim hopes to sell his science fiction drawings to a publisher. But they never get there. While on the road, Leo and Tim see a strange circle of lights in the sky, and before they know it, creepy, tiny-headed creatures are taking samples of the boys' blood on some sort of spaceship. Suddenly, Leo is back in his car and Tim -- along with Leo's memory -- is gone. Worse, when Leo finally starts to remember what happened, no one believes him. Before long, the 2 friends find themselves in the middle of a cosmic contest between warring alien races -- and the people of Earth are caught in the crossfire. Time is running out, and it's up to Leo and Tim to save the planet from catastrophe.