The book universe is a jumbled mess that is hard to make sense out of, so it's highly recommended to ignore the books altogether. This
page of
The Lion King: Serialized Storybook states the animals came to celebrate the "birth of the King's new son". I own a copy of
The Lion King: A Read-Aloud Storybook that was published by MouseWorks in 1999, and the ending page states "Soon the animals gathered at Pride Rock to celebrate the birth of Simba and Nala's cub." Note how it doesn't give the cub a specific gender. (Hopefully, I will be able to scan that particular page.) I also wouldn't be surprised if the current
Lion King storybooks on sale are just re-prints of the older storybooks that were sold in the mid to late 1990s.
Now, to answer your question, there is a storybook that describes "Fluffy" as a female. Here is the
scanned page of a
Lion King storybook. And Andrew Collins (the lead animator of Kovu) confirmed in an Australian edition of the
Disney Adventures magazine was asked "What's the major difference between this film and the original?" He answered, "It takes off where the first one left off with a twist - you think it's a king but it's a queen!" Click here for the article:
Africa Down Under.