Here and there in the book, he mentions books and writers that influenced him and on his recommendation, I have downloaded a book (circa 21 pages) called The Specialist by Charles Sale. I have not read it yet but I'm intrigued. If it is good enough for Sir Terry, then it is more than good enough for me.
Here's the blurb for The Specialist, from the Amazon site:
Even before I read it, I know it is going to be £1.25 well spent.