I think, in general, books in a series do motivate young readers to read more. It is easier to read when you are interested in the characters. A good series will drive you to finish reading because you want to know what happens to your favorite characters. Readers can look forward to the next book. Once they finish the first book, there is still more to read. With a single book, the reader is sent looking for another book after they finish reading. Often, this leads to a lull in the person's reading habits because they don't have a drive to continue reading a particular book. Series can deter young readers in one sense, because they can seem intimidating. Longer series with larger books (like Harry Potter for instance) might seem overwhelming to a young reader. A person might avoid reading the first book because they don't want to have to read all the books in the series.