I’ve started learning C four years ago, after a career spent coding ruby/javascript/Go, so I guess I was in the same position than you. I started by reading the GNU C manual, then the Glibc documentation, and the man pages for headers and functions I was using (all glibc functions have a man page ; depending on your distros, it may already be installed or you may need to install a specific package to install them all at once). And of course, reading header files in /usr/include/ is always useful for using this or that library. Later, I discovered the book Modern C and it completed my education to feel at home with C. I can’t say if it’s the best path, but it worked for me.