One of the popular reasons authors give away books for free is to build a readership, and to increase the sale through of other books in their series. (angelajford.com)
When readers download a free book and like it, they tend to buy other books in the series. (angiesdiary.com)
Free promotions seem to work particularly well for books in series, drawing people to buy other books in the series. (digitalbooktoday.com)