[dhimes]

To your point 1- You are also likely to come across texts that don't do a very good job explaining some fundamental/foundational ideas, perhaps because they were explained so well in the original papers- the ones that your textbook author read- that your textbook author didn't think it was worth spending too much time on it. The original source can be a goldmine in these cases.

This also happens between editions of textbooks by the same author. On an update they leave out stuff to save space or something.