What is "required reading" in training that describe mechanisms behind learning?
The questions you list are incredibly broad and diverse. You will find basic textbooks that may touch on aspects of all of these, but otherwise will not find answers to them all in one book. Some may not be answerable at all at the textbook level; others like &quot;How does myelination affect how babies learn?&quot; I feel are not really well-posed questions but some background will help you understand why.
I'd recommend starting with a broad undergraduate-level neuroscience textbook. These will assume you have some prior knowledge of the basics of molecular and cell biology, as well as physics (electricity and magnetism in particular; at least enough to be comfortable with basic circuits: voltage and current, resistors and capacitors, etc).
The two I would recommend (pick just one or the other) are:
Neuroscience, by Purves et al
and
Principles of Neural Science, by Kandel et al