For me, one of the most helpful guides to prayer is Luther's, A Simple Way to Pray, especially the edition with comments from Archie Parish. A Simple Way to Pray is a collection of Luther's pastoral advice on prayer. It came about when Luther's barber asked him for some guidelines on prayer. Luther's counsel on prayer was that we use the Lord's Prayer, the Psalms, and the Ten Commandments as well as the Apostle's Creed as models and guidelines for how to structure our prayers. Luther isn't bound up in rote rituals within these guidelines, but rather Luther believed that there were Biblical patterns of prayer that are helpful for all believers. Luther's guide is extremely helpful especially in it's practicality.
You can order that book here.