Stuttering is a mental illness, a neuropsychological one. Mental illnesses are recognized by cognitive/emotional/behavioral/social abnormalities (in that it's not part of the norm, not in a derogative way) causing significant distress and/or functional difficulties. This is interesting, it's the first time I've heard of primary and secondary stuttering. It's not that clear cut for me (I think it's not either for most, maybe this idea is outdated since the book is old), my mother stutters but I didn't stutter at first, I spoke early and well for my age, I've had selective mutism from preschool at least (very different from stuttering, you can't even activate the impulsion to speak and you can't translate your mind in words) and I began stuttering lightly at 6 yo, it became worse with age. For the neurology, I read articles about the parts affected, I saw mentions of abnormalities in a lot of areas (motor, coordination, sensory, speech), an imbalance between the left and right hemispheres (structural differences or reduced connectivity), and the limbic system (emotional) such as the amygdala, that is hyperactive and disrupts fluid motor control even more. It's all very interesting but complicated, I hope we learn more about it all in the future.