When the noun that noir modifies is masculine as in le livre then the adjective takes the masculine form noir . If the phrase was a black woman then it would be une femme noire because the adjective must agree with the noun. Une agrees with femme and so does noire.
Hi Melsin, in general, we use "mon" to describe nouns that are masculine (e.g. Mon livre est bon) unless in cases when that noun begins with a vowel (e.g. Ma animal est bon). We always use "Ma" to describe nouns that are feminine (e.g. Ma pomme est bonne).