The Google search included all categories - for all time. Rush's books did make #1 on the NYTimes Best Seller list, but have hade no where near the overall sales of other publications. The 'little red book' from Mao had a lot of sales in the 60's & 70's. Other books in the top 10 were books like 'The World Almanac', Guiness Book of Records', and a Spelling book.
Just making the Top 10 for a couple of weeks/months doesn't start to compare to sales over a number of years.
How about Macciavelli's The Prince?