https://www.ccel.org
Christian Classics Ethereal Library has a vast number of free books on prayer. If you know the title of the book, you can type the title in the search engine. If not, you can just put in the key word "prayer."
If you have any questions or need help finding a particular video, dvd, tract, book or any other Christian resource in any language of the world, please contact brianformissions@gmail.com
Christian Prayer Resources
http://christianprayerresources.org/
https://twitter.com/PrayerResource
https://www.facebook.com/Christian-Prayer-Resources-179361952653514/