Singular and plural nouns

May, just to clarify what Lewis has correctly said, you don't need to use a quantifier, but you can.  She is holding books is a perfectly acceptable sentence.  It would be most natural in a situation where you are describing what someone is doing, or identifying who she to another person. 

A:  Do you see that pretty girl over there?

B:  No. Which one?

A:  She is in line at the library counter.  She is holding books.

B:  Oh, right.  Yes, she's lovely!

You didn't ask about this, but you would need to use the article the if you are talking about a specific group of books which have already been mentioned:

A: I bought three books this morning, and I left them on the coffee table.  Now I can't find them.

B:  Hmm, I am not sure but, oh wait, there's Jane standing at the photocopier.  She is holding the books. 

It isn't needed with most plural nouns. You can use a quantifier to show how many books.

She is holding a few books.

She is holding many books.

She is holding a stack of books.

In this, the 'a' modifies 'stack' and 'books' becomes the object of the preposition.