Isaac Watt's psalms and hymns revolutionized the worship of the churches in the 18th century. He is better known for those hymns than for his other writings. This Guide to Prayer, however, has been termed "the best" by leaders from his time to ours.
Isaac Watts was an English hymnwriter, theologian and logician. A prolific and popular hymnwriter, he was recognised as the "Father of English Hymnody", credited with some 650 hymns. Many of his hymns remain in use today, and have been translated into many languages.
Watts was the author of a text book on logic which was particularly popular; its full title was, Logic, or The Right Use of Reason in the Enquiry After Truth With a Variety of Rules to Guard Against Error in the Affairs of Religion and Human Life, as well as in the Sciences. This was first published in 1724, and its popularity ensured that it went through twenty editions. Isaac Watts' Logic became the standard text on logic at Oxford, Cambridge, Harvard and Yale; being used at Oxford University for well over 100 years. [wikipedia]
This is the source for the ACTS model of prayer. Watts' goes even deeper, and his writings have influenced many since. This is a great devotional resource, though I read it as research for a class.