Choose a few books that you would like to read. It is best to choose books of different genres, in order to keep you focused and interested. As a suggestion, choose a non-fiction, a fiction, and a topic of interest to you.
Begin to read your first book with a goal of reading at least a chapter, with maximum time reading set at 20-30 minutes. The average adult attention span is about 20 minutes, so this is a good stopping point.
Switch books. Again aim for at least a chapter, or 20-30 minutes of reading. The new topic and reading style should help to increase your focus and attention span(that otherwise would have continued to diminish if you continued reading the same book).
Continue this pattern until you tire and need a break. When you are finished, you should have found you have covered a lot more total reading in one sitting, than you would have been able to with only one book.