Mystical and Spiritual Meaning:
Neale and Littledale, Commentary on the Psalms (4 vols.). This has a very valuable Introduction on the Psalms as used in the offices of the Church, and a dissertation (in vol. i.) on the mystical and literal interpretation of the Psalms.
Walpole, The People's Psalter.
General:
Prothero, The Psalms in Human Life.
Marson, The Psalms at Work.
Church, The Sacred Poetry of Early Religions, in The Gifts of Civilisation. The Discipline of the Christian Character, iii.
Robertson Smith, The Old Testament in the Jewish Church, lect. vii.
Orr, The Problem of the Old Testament, c. xii.
Alexander (Archbishop), The Witness of the Psalms to Christ.