The body of the screen is a list of the available hymn books.
Select a hymn book by clicking on it, and then clicking the "Ok" button.
Double clicking on a hymn book, does the same thing.
The "Is Default" box is ticked if the selected book is the default. If it is not ticked, clicking on it will make the selected book, the new default.
The "Help" button, shows this screen.
The "Cancel" button, closes the screen, ignoring any changes.
The "Ok" button, closes the screen, saving any changes.