Are you sure the book specifically said that those words were interrogative adjectives? If so, I think you ought to give us the title., so that people know that they must be wary of it.
Yes, I'm 100% sure it says this. It's only short. So short, I'll type word for word what it says:
................................................... Interrogative adjective An adjective (or determiner) that asks a question. These are the so called wh- words + how: