Quoted from the Letter to Marcellinus:
All the books of Scripture, both Old Testament and New, are inspired by God and useful for instruction [2 Tim 3:16], as it is written; but to those who really study it the Psalter yields especial treasure. Each book of the Bible has, of course, its own particular message: the Pentateuch, for example, tells of the beginning of the world, the doings of the patriarchs, the exodus of Israel from Egypt, the giving of the Law, and the ordering of the tabernacle and the priesthood; The Triteuch [Joshua, Judges, and Ruth] describes the division of the inheritance, the acts of the judges, and the ancestry of David; Kings and Chronicles record the doings of the kings, Esdras [Ezra] the deliverance from exile, the return of the people, and the building of the temple and the city; the Prophets foretell the coming of the Saviour, put us in mind of the commandments, reprove transgressors, and for the Gentiles also have a special word. Each of these books, you see, is like a garden which grows one special kind of fruit; by contrast, the Psalter is a garden which, besides its special fruit, grows also some those of all the rest.
You see, then, that the grace of the one Spirit is common to every writer and all the books of Scripture, and differs in its expression only as need requires and the Spirit wills. Obviously, therefore, the only thing that matters is for each writer to hold fast unyieldingly the grace he personally has received and so fulfill perfectly his individual mission. And, among all the books, the Psalter has certainly a very special grace, a choiceness of quality well worthy to be pondered; for, besides the characteristics which it shares with others, it has this peculiar marvel of its own, that within it are represented and portrayed in all their great variety the movements of the human soul. It is like a picture, in which you see yourself portrayed, and seeing, may understand and consequently form yourself upon the pattern given.