According to the February 2011 Frontiers in Psychology article, "Get the story straight: contextual repetition promotes word learning from storybooks", children who read the same stories repeatedly are more likely to increase their vocabulary as a result. In the study, 3-year-olds were either read the same story three times or were read three different stories. Each story contained the same novel (made up) words. Children who heard the same story over and over learned the novel words and retained the words over the long term, thus increasing their vocabulary, whereas the kids who were read the three different stories did not.In addition to allowing your kids to choose the same book night after night and reading it to him two or three or four times in a row some days, you can choose books that are written to emphasize repetition.
For suggested books that use repetition, visit Cookie's Book Club.