The section on prayer in Calvin's Institutes is one of the best I've read.
A Method for Prayer by Matthew Henry. Easily available on-line.
The Lord's Prayer should not be neglected. I think the Westminster Larger Catechism is very good on it, as well as the book the Lord's Prayer by A.W. Pink.
Best book on prayer outside Scripture is Nathaniel Vincent's, "Spirit of Prayer". It was heralded by the puritans in their day as "the" work on the subject.
I've been reading Prayer and the Knowledge of God by Graeme Goldsworthy, because it was lying around Ruben's study. It's very good.
I also love Paul Miller's A Praying Life, though that is a more popular memoir approach.
A Guide to Prayer by Isaac Watts; reprint (Edinburgh: The Banner of Truth Trust, 2001), 186pp. Originally published in 1715.
How to Pray: Reflections and Essays by C. S. Lewis (New York: HarperOne, 2018), xvii + 157pp. A collection of Lewis's thoughts on prayer gleaned from his various writings.