There are 66 books in the bible, unless you understand the Minor Prophets to be one book rather than twelve, in which case there are only 55, or if you understand Psalms to be five books rather than one, in which case there are 70.
But, let's stick with 66. One of those books is Psalms. I'm planning to preach a sermon on Psalms this week, having preached on Job last week.
We tend to think of Psalms as a collection of songs that reflect the different experiences of David and others. It is certainly that; but it is not a random collection. Those songs have been crafted into a book with a coherent message.
We tend to think of Psalms as being primarily about David; he is certainly in focus. But we must also recognise that Psalms is a post-exilic book. The focus is on David, because he is the model of the Messiah to come. Those who had returned from the exile and had not seen all the promises made to David come true. The return from exile has not marked the coming of the Messiah.
Psalms holds out an enduring Messianic hope to a post-exilic people. There is yet a rest to come.