Psalms

If it is even possible to measure and weigh such a thing, to me the Book of Psalms is one of the greatest books of scripture. It has everything: prayer, song, worship, praise, prophecy, even Messianic prophesy, and deep spiritual revelations. Every time I open the Book of Psalms, I find something new and amazing.
King David did not write all of the psalms, but he organized the use of psalms in worship. As the Israelites carried the ark of the covenant up to the tabernacle in the City of David, David, after consulting God regarding the proper order, assigned the singers and musicians:
"Then David spoke to the leaders of the Levites to appoint their brethren to be the singers accompanied by instruments of music, stringed instruments, harps, and cymbals, by raising the voice with resounding joy."  1 Chron. 15:16
The instruments appointed by God to David to be played with the psalms were just as important as the voices that would sing them:
"Moreover David and the captains of the army separated for the service some of the sons of Asaph ("gatherer", gather/harvest, to gather in, take away, assemble, to be brought in, together), of Heman ("faithful", believe, assurance, trust, steadfast, established, verified, made firm/sure/lasting, pillars, be carried by a nurse, confirmed, stand firm, to be certain) , and of Jeduthun ("praising", celebrating, make confession, give thanks, cast out, to revere or worship with extended hands, to throw a stone, shoot an arrow), who should prophesy with harps, stringed instruments, and cymbols..."  1 Chron. 25:1
These specifically chosen men and their sons following, were to prophesy on their instruments. Several psalms, according to traditional knowledge, had instrumental breaks ("higgayon/higgayon sela") amid the singing so the instruments alone could prophesy (examples: Ps. 9:17, Ps. 92:3-4).
Regarding the presence and importance of stringed instruments used for the Psalms, it is interesting that in Rev. 14, those who follow the Lamb, Jesus, wherever He goes have the voices of harps (v. 1-5). The harp is the voice of the redeemed! Since the instruments of the Psalms were given to David by God, and are called "the musical instruments of God" (1 Chron. 16:42) we can rejoice that we, the redeemed of Christ, known to God before the foundation of the world, were included in the Psalms, and are voicing our prophetic praise within them.
Even the musical instructions at the beginning of some of the Psalms carried great significance: 
Psalm 9: To the tune of "Death of the Son" or "To Die for the Son"/Muth Labben
Psalm 22: "The Deer (or great ram) of the Dawn (dayspring, rise, early, seek early, to look for diligently with painstaking care)"/ Aijeleth Hashahar
Psalm 45, 69, 80: Set to "The Lilies" (for whiteness and trumpet shape, to rejoice, exult, to be bright, be glad, to leap, to spring / Shoshannim, A Song of Love
Psalm 56: Set to "The Silent Dove in Distant Lands (or strangers)/ Jonath Elem Rechokim
Psalms 57, 58, 59, 75: Set to "Do Not Destroy" or Thou Must Not Destroy, Corrupt/ Al Tascheth
Psalm 60: Set to "Lily of the Testimony (evidence, testimony, witness)"/ Shushan Eduth
Often, Jesus has been called "The Lily of the Valley" among Christians, and there was also a hymn written with that title. He is mentioned as a "lily" prophetically in Song of Songs 2:1-2.
The apostles encouraged the believers in Christ to sing psalms (Eph. 5:19, Col. 3:16, James 5:13). It is believed that the "hymns" that Jesus sang with His disciples after the Passover Meal, or Last Supper (Mt. 26:30), and the songs sung by Paul and Silas while they were in prison (Acts 16:25) were the Psalms 113-118, 136, which are the Pascal Psalms, also called "The Great Hallel", sung after the fourth cup of blessing in the Passover Seder.
The Apostles quoted from the Psalms in their teachings to the Church. One such quote shows us the riches and depth of the Psalms:
"...to each one of us, grace was given according to the measure of Christ's gift. Therefore He says, "When He ascended on high, He led captivity captive, and gave gifts to men." (Now this, "He ascended" - what does it mean but that He also first descended into the lower parts of the earth?...)  Eph. 4:7-9 
Paul quotes Psalm 68:18 above, and revealed that this verse refers to the Messiah. From this verse in Psalms, Paul reveals to us that Jesus descended into Hell, and then took those held in Sheol out with Him when He conquered death, Hell, and the grave, before He ascended to His Father. The Apostle Peter also tells us that Jesus preached to the captives in Hell (1 Pet. 3:18-19).
When we go back to that verse in Psalms, and look at it in the original Hebrew, we discover something amazing. The root word for the phrase "led captivity" (shebeh/shebun) means "a precious stone on the breastplate of the high priest". It was a stone with divisions of "flashes and flame". The Book of Hebrews makes clear that Jesus is our High Priest, not a mortal high priest like the Levites, but an eternal High Priest, after the order of Melchizedek, who had no beginning and no ending (Heb. 5:9-10, 6:19-20, 7:1-3). This verse in the Psalms not only tells us that the earthly Levitical high priest wore a stone on his breastplate that represented the captives of Hell, but also that this stone prophesied Messiah's deliverance of those captives. This verse reveals the truth that our eternal, heavenly High Priest must also have worn this priestly breastplate and captivity stone from eternity, since the earthly things were patterned after the heavenly (Heb. 8:4-6). Jesus entered Hell and ascended with the captives as part of His role as High Priest forever. This is only one example of the depths found in just one verse from the Psalms.
The reading of the Psalms is considered so important, that groups called, "Hevrot t'hillim",  societies of Psalm readers, divide the Psalms among themselves and read them daily at the Western Wall in Jerusalem. Every day, the rich prophetic prayers and worship contained in the Psalms are spoken on earth, as they exist in heaven.
I encourage you to read and meditate upon the Psalms and all the scriptures.

Our Father has placed eternal depths of truth in the Psalms.