You can't just read a book, write the code they ask you to write, and learn C, or any programming language that way for that matter. You'd get tired of it, as you are now, and that is, or would be normal for most people. You have to try experimenting with the concepts on your own. Try creating interesting things with the concepts you learned, then move to the next project within that book perhaps. That book isn't going to teach you everything about C, so I don't see any harm in using external resources and experimenting while you're going through that book.