As I was waiting for the Yom Kippur service I attended this morning (outdoor, masked, socially distanced) to begin, I was reading the first couple of chapters in the Mishnah from tractate Yoma, describing the service of Yom Kippur by the High Priest - some of which was read as part of the Seder HaAvodah in the actual service. One thing that struck me was that the night before, he would study Torah (or listen to Torah scholars expound) and also read or listen to the reading of the books of Job, Ezra/Nehemiah, and Chronicles. One tanna added that he would also read from the book of Daniel to the High Priest. (And the Yerushalmi added Psalms and Proverbs to the list - but other than observing that Tehillim/Psalms is my favorite book of the Tanach - I even have a small copy that I carry in my wallet - and Proverbs is probably my LEAST favorite, I will not comment on that since it is not in the text of the Mishnah itself.) All are from the Ketuvim. Ezra/Nehemiah, Chronicles, and Daniel are not included in the synagogue lectionary - and Job is not by the majority of Jews, although Karaites and some Sephardim read Job on Tisha B'Av (Tenth of Av in the Karaite tradition).
The commentaries I looked at said that Job and Ezra/Nehemiah would depress the heart and make it hard to sleep (Bartenura), that Job and Daniel were compelling and interesting stories, and that Ezra/Nehemiah and Daniel were partly in Aramaic, so that they were easier to understand. Some think this last reason is a sad commentary on the lack of learning by the High Priests. The Meiri speculated that Daniel was included because of its concerns with eschatology and the hope of redemption.
But as I reflect, I see another reason for these four books to be included (it is possible that this is expounded in a commentary I haven't consulted, so please forgive me if this is an obvious point others have covered). These four books from the Ketuvim in some ways cover the breadth of human experience. Job is considered in rabbinic tradition to be a non-Jewish scholar (the Hebrew has some peculiarities leading to much scholarly speculation about it being a dialect of Hebrew spoken by a non-Jewish but closely related people, perhaps the Edomites). Chronicles focuses on the history of the kingdoms of Israel and especially Judah until the exile. Daniel is a Jewish sage exiled in Babylon. And Ezra and Nehemiah led the return of Israel to the Land after returning from the Babylonian exile.
There is something profoundly beautiful to me about the idea that the High Priest in some sense carried something from each of these disparate groups of people in his heart as he performed the service of the day - including his entrance into the Holy of Holies and his utterance of the Holy Name.