The Hymnal of the Mother-Beast is the holy text of the Sisters of the Mother-Beast. It is a hefty tome, hundreds of years old, with long cloth bookmarks.
Virtue came upon it on a lectern in a small side chapel in the Priory of the Mother-Beast. She flipped through it looking for information, and took it with her.
During her downtime in Episode 13, Virtue spent some time studying the book and taking notes, which were of little use.
Virtue gave it to Arbogast as payment to heal her major Blood fallout Downed.