Why did George throw the book into his own suitcase?

In Silver Streak (1976) after spending the night with Hilly, in the next morning when George finds out about Professor Schreiner from the photo in his book and can't convince Hilly about what he saw the night before he gets irritated and throws it into his own suitcase.

It wasn't a random throw because he stands still for a second and looks at his suitcase. Why did he throw it into the suitcase? It was Hilly's book and he particularly expressed his lack of interest about it the night before. This looked like a dumb plot twist to me because later in the movie the book became important.