Read the following paragraph:
There's a very obvious place to start for those wishing to improve their reading skills: books! Books not only help you learn a new language. They're also a way to discover the culture behind the language. Thanks to the Internet, paper books aren't your only option for reading practice. Most languages have thousands of books available to download online - many of them free.
The above paragraph refers to one of the four basic language skills. Which language skill does it refer to?