Primary language acquisition is the technical jargon for the process by which we all learn our native language as children. The book is far from being out on this one, as it directly confronts an issue of some serious contention in the modern linguistic community: is language a unique innate skill of the human mind, or simply one of many abilities granted us by our general cognition?
Children begin acquiring language very early. There have even been studies to claim that prosodic elements (stress and tone patterns) are learned while still in the womb. Preverbal children can tell their native language apart from others, and often babble differently from babies who "speak" another language. Some children begin speaking before their first birthday, others may take until their third birthday, though regardless of when they first started their linguistic abilities level, so while parents often prize an early starter its not actually that impactful.
People often argue about the effect of "baby talk" on children, whether it is good or bad for their growth. Some people speak to children, some people coo, in some cultures you barely acknowledge them at all until they can speak. In all these environments children acquire language just fine. In fact, children acquire language better from their peers than their parents, likely a biological assurance that you speak the language of your peers and not the language of prior generations. Children are so good at learning language its basically impossible to stop them; they can even generate language from non-language, like creoles out of pidgins or sign languages from hand motions.
One of the central idea on the "language is innate" side is called the "poverty of stimulus”. Simply put, this argument insist language must be innate because the stimulus we receive as kids is too poor to just figure it out without instinctual aid. Consider that children are not growing up in classrooms laying out language for their ease of consumption. They have to make do with what they hear, which includes lots of incomplete utterances and other less than ideal demonstrations. You might think that children then rely on constructive feedback from adults, but this is unlikely. Not only is this just not how most adults around the world speak to children, but even if it were correcting children often results in… nothing. I recall one well known example where the child asks for "the other one spoon", to which the adult corrects to "other spoon" and the child responds "yes the other one spoon". This goes back and forth much to the amusement of linguistics students around the world. That example also raises another point of interest with children: they say things they didn't hear. No one taught, even inadvertently, that child to say "other one spoon". He pieced the grammar together himself.