I had trouble understanding 2's complement using textbooks alone, so I used YouTube instead. There are 10 minute videos that'll make you understand the system better than any textbook.
If your textbook has exercises about it somewhere I recommend you do a couple to get acquainted.
Before learning to code you should probably have a better understanding of computers in general. The book I used (and lots of universities use) is called "Computer Science: An Overview" by Glenn Brookshear. It covers a lot of topics that anyone interested in CS should know, including binary representations, among other cool things.