One of his most famous works, "The Age of Imperialism", his first and arguably most influential book, came out in 1969. The book sold over 100,000 copies and was translated into fifteen languages. Two years later after the death of Leo Huberman, Magdoff began co-editing the "Monthly Review" with Paul Sweezy, and continued to edit the magazine into his 90th year. Magdoff and Sweezy together produced five books, as well as many years of "Monthly Review". One of Magdoff's last books was "Imperialism without Colonies", published at age 89. "Monthly Review" is one of the preeminent socialist journals in the world, a journal characterized by its independent, nonsectarian Marxist approach.