some numbers to ponder

Take a close look at this table, which tabulates sales data for Samuelson's and McConnell's texts between 1948 and 1984. 

From Kenneth Elzinga, "The Eleven Principles of Economics," Southern Economic Journal. Vol. 58, No. 4, 1992, pg. 874.

While you could argue that there were some isolated years in the mid- to late-1960s when McConnell outsold Samuelson, upon closer look at the edition release years you can see that it's not until McConnell's 4th edition -- in press from 1969-1971 -- that it outsold Samuelson's competing new text, the 8th edition -- in press from 1970-1972. The previous editions of the two texts were certainly competitors, but Samuelson won by a hair in the previous edition wars: 389,000 for his 7th edition vs. McConnell's 378,000.

Also, notice the dwindling sales of the 11th edition of Samuelson (and the long revision date! for a while, both texts were on a 3-year cycle). For the 12th edition, released in 1985, McGraw-Hill brought Nordhaus on board as a cowriter of the text in order to try to reorient the ship. One of the big problems with Samuelson's book was its explicit Keynesian focus -- no doubt a product of the seminal intellectual and political influence of its author, but something which in the end would harm a textbook writer, since one needs to be able to adapt in some way to the shifting political winds (particularly if you're writing in the mid-70s, early 80s!).