I am now finishing my work on a second edition of my 2003 book. I still have to be concerned with diagrams, page references, and lots of little things. But in editing a chapter just now, I discovered that I had not italicized m or n, referring to numbers, and other such minor features, and other getting other symbols just right.
I believe there must be some people in the first 90% of the work, actually have only 10% more work to do, but I am not one of them. I discover new things I am not sure I really understand, various points I need to check, and I am still unsure if chapter 4 should stay as it is, or be split into two chapters.
These are of course nice problems to have. You have an earlier book already, someone wants to put out a second edition, and you have a sense that you can take care of most of the issues one-by-one. But, I keep thinking of those 90%/10% folks--although I do not know anyone who claims to work this way. In fact, I have little idea of how any of my scholarly friends write and edit and rework stuff. It's the big secret of the academic life, since we write in private for the most part. People are more likely to hint of their intimate escapades than how they actually work, although in fact you hear very little of either since people are more likely to go on about what's wrong with the institutions, its administration, and their colleagues. It's almost impossible to get them to tell you the argument of what they are writing or even its theme, unless you use instruments employed by oral surgeons to remove wisdom teeth with problematic roots.
When it's all done, I'm not sure what to do. It's out of my hands. I've done what I could. Time to move on.