Last year I read through this little book and really enjoyed it. I noticed it is available on Monergism books in kindle format for US$2.45 which is chump change.
This is a very practical book that does a great job of pulling together the writings and practices of some of the great Christians throughout history including Charles Spurgeon, Jonathan Edwards, David Brainerd, Martin Luther, George Whitfield and George Muller.
The book is organized as you’d approach prayer: beginning with composing our mind, then dealing with worship, confession, requests and finishing with how we can expect God to answer.
If you are looking for some encouragement in prayer, this is a great book for that purpose, and it’s short and easy to read.
If you don’t have a Kindle, you can either buy one (support this site by buying through this link) or purchase the paperback version instead.