Seriously, I had to read this with a lot of self control, because there was so much bullshit. I mean, you don't need to know Nginx to understand that the PID file is used to know if the process is running (in the most simple case) but the book tries to sell it as a way to check process uptime. I mean, seriously? Of all the things, Nginx will create a PID file simply for you to check its uptime?
This kind of thing keeps going on and on. It feels like the author didn't know Nginx at all and was figuring out as the book was being written; when they had no idea what one thing meant, bullshit was used to fill the blanks.
This means two things: One, there is so much bullshit it is painful. Second, the book only gets reasonably good at the end. But still hardly anything near the "Pro" as used in the title.