I would figure that the book had an invisible sign--"no non-Hispanics need apply"

I disagree with this. For one thing, there are Latinos who don't speak Spanish and Spanish speakers who aren't Latino. But also, as others have said, this is a book written in English with some Spanish words and phrases that you can look up in a dictionary or on the Internet if you want to get all the meaning you can. She's writing about her culture and part of that culture is bilingualism and codeswitching, so it makes sense to me that that would be part of her writing as well.