The Dragon And The George by Gordon R.Dickson. From Amazon's description:
"One minute he was a college teacher, the next he was a dragon! Jim Eckert leads a peaceful, happy campus life. He's a college basketball player and an expert in mediaeval history.
But his life has been disrupted by the disappearance of his girlfriend Angie. While taking part in a laboratory experiment on astral projection she just vanished.
In an attempt to find out what's happened he submits himself to the equipment. A very unwise move Next thing he knows he's in the Middle Ages. Worse, he's no longer human. He's in the body of a dragon. Worse still, Angie has been nabbed by another dragon and taken to the Loathly Tower. Obviously, Jim has no option but to give chase . . .
