2 Now in that some have believed that the Psalms are divided into five books, they have been led by the fact, that so often at the end of Psalms are the words, “so be it, so be it.” But when I endeavoured to make out the principle of this division, I was not able; for neither are the five parts equal one to another, neither in quantity of contents, nor yet even in number of Psalms, so as for each to contain thirty. And if each book end with, “so be it, so be it,” we may reasonably ask, why the fifth and last book has not the same conclusion.
We however, following the authority of canonical Scripture, where it is said, “For it is written in the book of Psalms,” know that there is but one book of Psalms. And I see indeed how this can be true, and yet the other be true also, without contravening it. For it may be that there was some custom in Hebrew literature, whereby that is called one book which yet consists of more than one, just as of many churches one church consists, and of many heavens one heaven,...and one land of many lands.
For it is our everyday habit to say, “the globe of the earth,” and “the globe of the lands.” And when it is said, “It is written in the book of Psalms,” though the customary way of speaking is such that he seem to have wished to suggest that there is but one book, yet to this it may be answered, that the words mean “in a book of the Psalms,” that is, “in any one of those five books.” And this is in common language so unprecedented, or at least so rare, that we are only convinced that the twelve Prophets made one book, because we read in like manner, “As it is written in the book of the Prophets.” There are some too who call all the canonical Scriptures together one book, because they agree in a very wondrous and divine unity....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)