Not every Bible reader realizes that the Book of Psalms is in itself divided into five books, and this volume deals with the middle three. It does not have much introductory material, but what is included is very good: following Gerald H. Wilson, Johnston sees an overall storyline in the Psalter, in which the Davidic monarchy is celebrated (Psalm 72, at the end of Book II) but then abandoned (Psalm 89, at the end of Book III). Book IV then focuses on God's power and authority and David indeed makes a comeback.
In keeping this structure in mind, Johnston is particularly good at looking at the psalms in the context. For example, he very helpfully explains how Psalm 84 follows the section 73 - 83 (p. 390). There is very little reference to the Hebrew language in the commentary, but Johnston's exegesis is solid.
The chapters in the book (one per psalm) have a sermonic feel to them - each one starts with an illustration or anecdote. I noticed a rather American flavor with many of them. But it does mean there is a great emphasis on application, more so than the other volumes of this series that I've read.