I would say "isang cover ng libro."
By the way, we don't often make a distinction between 'a' and 'the' especially in the spoken language so if you're going to construct a sentence using that, it would sound as if it was taken from a grade school story book. (We do use "isang" to express the number instead of definiteness).
Here's an example sentence:
Cover ng libro 'yan. -That's a cover of a book.
These days, "balat" is only used to say "skin," "peel," or perhaps "outer layer." I think I remember learning in school that it's called "pamalat" but vaguely, I don't call it that. I just say "cover."
"Aklat" also is hardly ever used nowadays except maybe in writing. Both the Spanish "libro" and the English "book" have taken over.
I hope you find this useful.
