Number

I agree with Lewis...  There is one more possibility, though.  If you're just amaking a flat statement of fact without a question, you really should consider the impact of choosing the determiner.

When you use "one", you emphasize the quantity, that there is one and only one book.  When you use "a" (an indefinite article), you leave the book feeling much less defined.  What impact do you want to have on your reader?  If you're going for any sort of suspense, use "a", but if you want to create a sense of definite reality, then using "one" might be more approriate.