He addresses all of those questions in the book - including why he considers those the core exercises and why he uses the novice program that he does and when he thinks acccessory exercises should be added.
The book is largely how to perform the movement and why to do it - the little novice program that he mentions at the end is an example that he uses with his starting lifters. It isn't a book about the routine.
Try the FAQ at the
Starting Strength Wiki - it isn't Rippetoe but the guy answers as he would for the most part. And most of your questions are there. And then if you have more questions, you can ask Mark in his forum at strengthmill.net or the archives there might give you some insights (over there, they really like you to have read the book first
)