I learned Unix by reading books, magazines and blogs. It was long time ago. One resource I still can remember and like is “Unix Programming Environment” by Brian W. Kernighan and Rob Pike. I read its Japanese translation from ASCII 1.However this book was aged even at that time and the original English version from Prentice-Hall seems discontinued. It is not something I could recommend to others.
A more detailed approach is to use the man command. Short for manual, man provides a manual page (called manpage) for a command you specify.
But new developers also should know the difference between
kill(2). How do people learn that nowadays?