Also, your questions at this point are extremely vague. You need to spend time determining exactly what you want to learn. Because unless you have a clear goal, you'll never progress. I'd start by researching how Nginx is used with whatever app you're using, and then start asking yourself questions like "why do this via Nginx instead of doing it with just the app?", then go find the answers to "why not use $app as the server." and get Nginx out of the mix, because the two are not directly related. Because where Nginx could be used, so could Apache + Passenger or some other stack.
- UNIX and Linux System Administration Handbook
- The Practice of System and Network Administration
- The Practice of Cloud System Administration
- Tannenbaum's textbooks (operating systems / networks)
- Readings in Database Systems