What you realise when writing a book is that every sentence gets rewritten 3 times, even before you start caring about proofreading. With all the research and decision making, publishing etc. it takes much more time than it looks.
However, even though I'm making quite a few changes/additions including a new chapter, it's a lot easier to patch something that exists than starting with a blank page. The fact that I don't need to churn out a huge number of words lets me focus on the new material and refactoring as needed.
When I was an industry analyst, I used to joke that the effort to write something went up by something like the square of the page count. There are counter-examples but mostly I think it's at least somewhat true given that you need to sort of keep your head around the whole project.
And I agree with the blank page... The more of the book is written, the easier it gets (I think).