The 12th Planet is a book by Zecharia Sitchin, first published in 1976. It is the first book in Sitchin's 'The Earth Chronicles' series, where he interprets ancient Sumerian and biblical texts to propose a theory of ancient astronauts and a hypothetical planet called Nibiru, inhabited by a race called the Anunnaki who allegedly visited Earth in antiquity.
This is this guy Zachariah Sitchin. It's fascinating stuff.
""Zecharia Sitchin was mentioned in the context of Sumerian texts and the planet Nibiru, directly linking him to his most famous work, 'The 12th Planet'."