The Bible. Everything else is optional. Well maybe FR but that isn’t a book per se.
I read and listened on audio book to the Bible at least three times straight through. I read the daily readings from the USCCB. I have been through several books/themes of the Bible in Bible study. Right now we’re going through Isaiah. For a while I read the liturgy of the hours which is almost all Psalms. But it was too demanding and I prefer less structured prayer. As I get older I read more.