As for the other matters concerning Jehoram, all that he did

"To read more about the history of Jehoram and what he did,"

are they not written ... Judah?

This question is used to either inform or remind the readers that the information about Jehoram is in this other book. AT: "these things are written ... Judah." or "someone has written about them ... Judah." (See: rc://en/ta/man/translate/figs-rquestion and rc://en/ta/man/translate/figs-activepassive)