In all his work, he gave thanks to the Holy One, the Most High, proclaiming His glory (Sir. 47:8).

In the literal sense, these words are attributed to David, and it is also appropriate to take them to show the cause of this work. In these words we are shown four causes:the MATTER, the MODE or form, the END, and the agent (the AUTHOR).
The MATTER of this book is universal; while each of the books of the canon of Scripture have a particular matter, the Psalms contain the general matter for all theology; this is what Dionysius says in the third book of the Ecclesial Hierarchy: "The Sacred Scripture of divine songs, i.e., the Psalms, has the purpose of singing without end all the sacred and divine works." Thus, this phrase in all his work indicates the matter of this book, because it deals with the entirety of God's work.

Now God's work has four dimensions:

The work of creation: God rested on the seventh day from all the work He had done (Gen. 2:2);
The work of governance: My Father is still working (Jn. 5:17);
The work of reparation: My food is to do the will of Him who sent me and to complete His work (Jn. 4:34);
The work of glorification: The work of the Lord is filled with His glory (Sir. 42:16).

And this book teaches us:

First of all the work of creation: When I see the heavens, the work of Your hands (Ps. 8:3);
Then the one of governance, because all the stories of the Old Testament are recalled in this book: I will open my mouth in a parable, and reveal hidden lessons from the past (Ps. 78:2);
Next, the work of reparation, as to the Head, i.e., Christ, and as to all the effects of grace: I lie down to rest and I sleep, I wake, for the Lord upholds me (Ps. 3:5); actually, all that which concerns faith in the Incarnation is taught in this work in such a limpid way that one can almost call it a Gospel, and not a prophecy;
Finally, the work of glorification: Let all the faithful rejoice in the Lord (Ps. 149:5).

Thus if the Psalter is especially used in the Church, it is because it contains the whole of Scripture. It is also, according to the Gloss, to give us the hope of divine mercy: David, although he had sinned, was nevertheless entirely renewed and restored through penance.The matter of this book is thus universal because it is the entirety of God's work and because this work looks towards Christ--for in Him all the fullness of God was pleased to dwell (Col. 1:19)--this is why the matter of the book of Psalms is Christ and His members.

In what concerns the MODE of this book, we know that one finds multiple modes or forms in Scripture:

A narrative mode: The Lord has empowered His holy ones to recount all His marvelous works (Sir. 42:17). We find this in the historical books;
A mode of warning, of exhortation, and of precepts: Declare these things; exhort and reprove with all authority (Titus 2:15); Remind them of this, and warn them before God (2 Tim. 2:14). We find it in the law, the prophets, and the books of Solomon;
A mode of argumentation in the book of Job and the epistles of the Apostle: I desire to argue my case before God (Job 13:3);
A mode of intercession or praise that we find in the Psalms: everything that is said in the other books according to the modes about which we have spoken is here presented by mode of praise and prayer: I will praise You, Lord, with all my heart; I will recount all Your wonders (Ps. 9:1). This is why Sirach says that David gave thanks, because he spoke by mode of praise.
This explains the title, "Beginning of the book of hymns or monologues of the prophet David about Christ."

The hymn is a praise of God accompanied by a song. And the song is the exultation of the mind on the subject of eternal things, springing forth and resounding in the voice. Thus the book of Psalms teaches how to praise God with exultation. As for the monologue, it is a one-on-one conversation of man with God or with himself, because this is fitting to the one who praises and who prays.

The END of this book is prayer, which is the lifting up of the mind to God: "Prayer is the ascent of the intelligence to God" (Saint John Damascene); The raising of my hands as an evening oblation (Ps. 141:2). Now the soul is lifted up to God in four ways:

In order to admire the lifting up of His power: Lift up your eyes on high and see: Who created this? (Isa. 40:26); How many are Your works, O Lord (Ps. 104:24). This is the lifting up of faith;
In the second place, the soul is lifted up to strive towards the excellence of the eternal beatitude: Surely then you will lift up your head without blemish, you will be secure, and will not fear; you will forget your misery... and your life will be brighter than the noonday! (Job 11:15,16a,17a). This is the lifting up of hope;
In the third place, the mind is lifted up to remain fixed and attached to divine goodness and holiness: Rouse yourself, stand up O Jerusalem (Isa. 51:17). This is the elevation of charity;
In the fourth place, the mind is lifted up to reproduce divine justice in a work: Let us lift up our hearts as well as our hands to God in heaven (Lam. 3:41). This is the elevation of justice.
These four modes are implied in these words: To the Holy One, to the Most High. The last two modes of lifting up correspond to to the Holy One, the first two to to the Most High.

And the Psalms themselves show us that the goal is found here:

Concerning the Most High: From the rising of the sun to its setting, praised be the name of the Lord. High above all nations is the Lord, above the heavens His glory (Ps. 113:3-4);
Concerning the Holy One: Let them praise His name, so terrible and great (Ps. 99:3).
This is why Saint Gregory says in his first homily on Ezekiel that the song of psalmody, if it is executed with the intention of the heart, prepares for Almighty God a way towards our heart, so that He infuses into the attentive soul the mystery of prophecy or the grace of contrition.

The end of this book is thus that the soul be united to God inasmuch as He is Holy and Most High.

These words of Sirach, proclaiming his glory, allow us to know the Author of this work. We must note that the case is completely different for Scripture and the other sciences. For the other sciences are produced by human reason, but Scripture is produced by the instinct of divine inspiration: No prophecy ever came by human will, but men moved by the Holy Spirit spoke from God (2 Pet. 1:21). And this is why the language of men behaves in Scripture like the language of a child speaking the words that another presents to him: My tongue as nimble as the pen of a scribe (Ps. 45:1); The spirit of the Lord speaks through me, His word is upon my tongue (2 Sam. 23:2). This is why Sirach says proclaiming the Lord or his glory, because they are said through Revelation--"Strike me," It the command of the Lord--i.e., through a divine Revelation.

One can say that this book proclaims His glory in four ways, because it is related to glory in four ways:

As to the cause from which it proceeds, because this teaching emanated from God's glorious Word: That voice was conveyed to him by the Majestic Glory, saying, "This is my Son, my Beloved, with whom I am well pleased" (2. Pet. 1:17);
As to its contents, because in this book is contained the glory of God which it announces: Tell the nations His glory (Ps. 96:3);
As to its mode of emanation: actually, glory is the same thing as clarity, and the revelation of this prophecy was glorious, because it was manifest;

There are actually three modes of prophecy:

By sensible realities: Immediately the fingers of a human hand appeared and began writing on the plaster of the wall of the royal palace, next to the lampstand. The king was watching the hand as it wrote (Dan. 5:5);
By imaginary similarities, as we see concerning Pharaoh's dream and the interpretation given by Joseph in Chapter 41 of the book of Genesis. And we also see this in the book of Isaiah: I saw the Lord sitting on a throne high and lofty (Isa. 6:1);
By the manifestation of truth itself. Such a mode is the one which fits the prophet Daniel, who by intuition from the Holy Spirit alone, without any outside help, pronounced his prophecy. For, as Augustine says, the other prophets prophesied facts and statements starting from images of realities and from the envelope of words, i.e., by dreams and visions; but he was simply instructed by truth.

This is why in saying the Spirit of the Lord speaks (2 Sam. 23:2) David also adds like the light of morning, like the sun rising on a cloudless day (2 Sam. 23:4). This light is the Holy Spirit illuminating the heart of the prophets; sometimes, it appears behind clouds and gives its light to the prophets according to the first two ways, sometimes without clouds, like here. We can here add this verse: How the king of Israel honored himself today, uncovering himself before the eyes of his servant's maids, as any vulgar fellow might shamelessly uncover himself (2 Sam. 6:20).

And because by this book He invites us to glory: This is the honor for all His faithful (Ps. 149:6). Thus it is correct to say how the king of Israel honored himself... (2 Sam. 6:20).

Thus the MATTER of this work is manifest: it deals with the entirety of God's work. Its MODE is a mode of intercession and praise. Its END is to lift us up so that we may be united with the Most High and Holy One. Its AUTHOR is the Holy Spirit Himself who reveals these words.