Absolutely this. I don’t know that I’ve ever seen any work of nonfiction that equals it. I had cursory familiarity with C in ‘94. I got this book in ‘97. I don’t even recall learning C because it was as if I just skimmed back and forth through the book a couple of times and presto. I knew C. I did one medium sized project with it, and then I got my first real programming job, writing C code among other things.