The Hobbit was released as a single book. Lord of the Rings was released originally in three volumes. That may all still count as one book in Europe but in simple America, a single book is any one draft you can read cover to cover thus the second and third volumes would be considered second and third books which is why three films is better fitting for that particular title. Not that I care Hobbit is being made into multiple movies, I just didn't prefer the ending.

As for the whole Part 1 thing... I despise it altogether. Harry Potter and Twilight were good stories, but they didn't need to expand it the way they did... they were just milking another movie out of the franchise.