"They read about the fall in a book."
Translation:De læser om efteråret i en bog.
It would probably be best if this said "the autumn" instead of "the fall", as I thought of the fall or decline of a regime or something like that, and duly translated it as "De læser om nedgangen i en bog". Nobody would mistake "autumn" for anything but a season!
But they wouldn't use a word that hasn't been introduced earlier.. And in this unit we are learning time expressions including the seasons! ;-)