[The Dragon And The George][1] by [Gordon R.Dickson][2]. From [Amazon's description](https://www.amazon.co.uk/Dragon-George-Cycle-Book-ebook/dp/B005LB9HWA):

> "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 . . .

  [1]: http://www.goodreads.com/book/show/114936.The_Dragon_and_the_George
  [2]: http://en.wikipedia.org/wiki/Gordon_R._Dickson