How can I prevent a downloaded Google Play Book from being purged?

I have purchased a digital book on Google Play. To read a book, the Google Play Books application downloads a copy to my Android device. This all works as expected. The problem is that I read this book only intermittently, and when I return to the Google Play Books application, sometimes the application needs to redownload the book. This is a problem in situations where data is unavailable (on transit, for example). I guess some kind of periodic cache purging is taking place. Are there settings that will prevent the book from being removed from the Android device? I have plenty (~5GB) of storage space available, and I would prefer that the book simply be saved until I manually remove it.