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?