The idea is to deal with the elephants in the room. Those door stoppers that you keep putting off.
Read the longest book on your TBR pile in each of the 12 categories. 
Twelve months, twelve books; hopefully more than 6,000 pages put to bed.

Don’t worry too much about the categories. Tweak the wording to suit yourselves. Slay those big beasts and have fun doing it!