You can't say "a prose" in English because it's an uncountable noun - to use the indefinite article you would have to say (e.g.) "a prose work".
What kinds of books is this word commonly used for, please? Is this how you'd refer to a novel? How about a non-fiction book, like a piece of academic research?
Πεζογραφία/Prose can be fiction or non-fiction.
It means it's not poetry.