What I've seen of KStar's stuff is good for functional strength, but for posture in general he advocates the more traditional, conservative PT approach of "just straighten the back", which has become controversial in more recent years as studies have shown that it's actually not that effective at reducing pain. I think Gokhale's book is a little more honest in not coming from any sort of academic background, even though I've met people who have exaggerated sway back postures and believe they are following the advice in her book.
What I've taken away from years of reading this kind of stuff, of dealing with back pain, and of exercise and sports is this: the body is meant to move in many different ways, and the ones you don't use, you eventually lose. Each joint has a certain range of motion, with muscles to support the full range. Correct posture is neutral and relaxed, and there is no one, fixed "proper" form - humans are built to move. You should be able to hold a J curve, an S curve, and a C curve in the spine with relative comfort, at least for a short while, and should put all of your joints through their full ROM daily, and gently.
What I've taken away from years of reading this kind of stuff, of dealing with back pain, and of exercise and sports is this: the body is meant to move in many different ways, and the ones you don't use, you eventually lose. Each joint has a certain range of motion, with muscles to support the full range. Correct posture is neutral and relaxed, and there is no one, fixed "proper" form - humans are built to move. You should be able to hold a J curve, an S curve, and a C curve in the spine with relative comfort, at least for a short while, and should put all of your joints through their full ROM daily, and gently.