What was the best-selling book of 1981?

The question doesn't specify fiction or non-fiction. Also, what source should be used for this data?

According to Publishers Weekly, the Number 1 book of 1981 was "Noble House" by James Clavell.
https://wiki2.org/en/Publishers_Weekly_list_of_bestselling_novels_in_the_United_States_in_the_1980s

The following link has the 1981 number-one books according to the New York Times bestseller lists.
https://wiki2.org/en/List_of_The_New_York_Times_number-one_books_of_1981

The following source claims that it was in fact a non-fiction book:

"The Simple Solution to Rubik's Cube" by James G. Nourse is a book that was published in 1981. The book explains how to solve the Rubik's Cube. The book became the best-selling book of 1981, selling 6,680,000 copies that year. It was the fastest-selling title in the 36-year history of Bantam Books.

https://en.wikipedia.org/wiki/The_Simple_Solution_to_Rubik%27s_Cube

Curiously, the Rubik's Cube book did not figure on the New York Times non-fiction list for 1981.