There seems to be a big trend toward authors publishing programming books for free online and offering hard copies that you buy, too. In general, if someone offers me a free product and a paid one, I'll stick with free. No surprise there. But coding books are an interesting exception.

I find it much easier to learn a new language when I'm using a book instead of reading it for free online. A book is a single-purpose device; you can only use it to learn about one topic, or set of topics. It doesn't vibrate, notify me of emails and text or encourage me to go on other tangents. For a goal such as learning R or Haskell, which requires a lot of mental energy, having this extra focus makes a big difference.