The book took me about 3 months to write. I started just before Christmas last year. There were also two months of editing and proof reading at the end. I can't belive how many times I reread each chapter :-) As an author, I'm sure you've experienced the same thing.
I wrote the book because I spoke to a lot of developers who weren't sure about XML and how they could use it in their applications. I wanted to provide a one stop reference and introduction to all things XML. I was also aware that a lot of the books around were a little dated and didn't address the latest browsers and scripting techniques.
I guess I see my book as a good introduction to XML. It gives readers a taste of everything including DOM scripting, Ajax and Flash. It's not specifically about one aspect of XML so it makes a good starting point before reading a book like Head Rush Ajax.
Out of interest, my book Foundation XML and Flash was published by friendsofed last year. It goes into detail about the Flash side of XML.