I'm not aware of any book specifically addressing the rivalry or both brands at once. My opinion is that this rivalry mostly exists in the press and in the public imagination and therefore would make poor fodder for a book. In contrast, Ferrari's dealings with Ford both on and off the racetrack are much more interesting, and have generated a lot of writing as well as the recent movie.

My #1 book recommendation would be Luca Dal Monte's excellent recent biography on Enzo Ferrari. In a field dominated by shallow coffee table books and breathless romanticism, it's a comprehensive, scholarly history that remains very readable. Not much material on Lamborghini, though. Unfortunately like many auto books it is now out of print and prices are in "collector" territory. Still worth picking up if you can.