Dr Pimsleur created the program that was bought by Simon & Schuster.
I first heard about Pimsleur when reading How to Learn any Language by Barry Farber. Back in those days nobody sold Pimsleur and it was quite a confidential product.
I spoke several time with M. Charles Heinle, Paul Pimsleur's partner and owner of Heinle & Heinle (now owned by Thomson), the company that first produced the Pimsleur tapes. He explained to be the great efforts and man hours that went into creating each program. They do not make all three volumes for every language since demand is greater for beginner courses and thousands of man hours are needed to make each new course, which is very costly.