There are many oilfield books in Kindle format (see in the screen capture). You can improve your skill by reading oilfield technical books.
Additionally, there are million kindle books in several topics.
Many people think that in order to read the kindle book they need some kinds of tablets as Kindle tablets, Ipad, Sumsung Galaxy Tab, etc. However, they can read the kindle books from the computers as well.
Firstly, you need to check the following links to download the Kindle reading program which is 100% Free.