This book holds up. It's a third memoir of King's beginning, a third writing advice, and a third memoir of his recovery (from addiction and from a near-death accident). Somehow the three strands hang together. And I think the trick is in how well they've been crafted.
For example, at the end of the first part, King describes his early alcohol and drug addiction with some humor. But then, as it gets serious, damaging, and himself withdrawn and damaged, the language and tone become uniformly flat. Almost void of emotion. That affectation is opposite most of the rest of the book and thus pitch-perfect. King on cocaine was flat.
Compare that to his earliest memories of family and turning to writing or his early and dirt poor marriage. Practically dripping with emotion. You feel very strongly you know something about what made him. Both are written in a spare style, but each section shows itself well.
Thus when he pivots to describing how one ought to approach writing, it's natural: you've been seeing him do it, now, he tells you how. The resonance of his methods are strong, as is the challenge of commitment he displays. On writing should scare you away from the craft, just a bit, because you see what a master brings and you tremble.