Looks better now. Though I have some nitpicky remarks as you might have expected:
- Regarding the last sentence in chapter 2: There's a dedicated character × in Unicode, you don't have to fall back to lower case x. As a technical writer you know that.
- For chapter 3, please see my remarks above. A little more visual structuring might help here, too.
- At beginning of chapter 4, you wrote: "And that too is a key point: you need to be aware of what's going on in the stack, even though you can't see all of it." I tend to disagree: IMHO one of the advantages of RPN is you don't have to care about stack details for most calculations as long as you calculate inside out. Well, it may become tight but usually works out well with four stack levels in real life. Eight levels would be better but the HP-35S doesn't feature them.
- At the end of chapter 4 I can't find the link to chapter 5.
- At the end of chapter 5 I can't find the link to chapter 6. So I guess your rework didn't reach these chapters yet.
At the bottom line, you started a journey - there are some miles to go still.
d:-)