The Accusative of plural «книги» is the same as its Nominative: «книги». This is the same rule that governs the choice of the Accusative form by neuter an typical masculine nouns. In plural, it extends to nouns of all classes: animate nouns use the Genitive form while inanimate nouns use the Nominative one.
Because книги is plural.
If you wondered why can it not be Genitive singular, well, книги-singular would seem a far fetched interpretation of unknown meaning, given that the forms are already the same. A book is not understood the same as "water" or "crap", which you might want an indeteminate quantity of. A book, a cat or a ball are objects with well defined boundaries, and whereas half a bottle of water is still water, a piece of a book is not called a "book".
However, it is indeed possible for substances and some abstract nouns:
- Я хочу воды.
- Я хочу покоя.