Definitively rude.
If you take the time to dine with someone, you must also take the time to converse with that someone - even if it is empty small talk. If you want to read - stay at home in your studay, or go to the library.
I agree that it is rude when dining with others, but how about when dining alone? I always take a book with me when dining alone. As a female, I hate dining alone.
Amen. Rude and boorish. Stay home.