It is possible that symptoms of inattention are more obvious while reading, especially if the child does not like to read or is not good at reading. However, it is most likely that symptoms of inattention would be present in other circumstances and settings and during other activities. This is a question for a pediatrician or developmental psychologist. Visit CHADD.org for more information.
It also may be that they are not interested in the text of the book, or that they don't like to read. If the symptoms in question are just slow reading or subvocalization or true vocalization during reading it may be a different disorder, or that they find reading for a while to be bothersome. Although, there are cases where the child would hyperfocus on the book instead and not notice surroundings due to being engrossed in reading. This can also be a symptom. I would ask a psychiatrist who specializes in children's psychology for more information, and do so in person if I were you, to have a better discussion.