Novelist Walter Wager wrote the seven I SPY novels, making Robinson and Scott agents for the CIA. That was never pinned down on the series. He also gave them a consistent boss handing out their assignments, something else the series went in a different direction with.
The two novels he did for MISSION: IMPOSSIBLE were the first and number four(the middle pair were by Michael Avallone I believe, under pseudonym of Max Walker. The first novel featured Dan Briggs(played by Steven Hill during the first season) and the last(all the other three for that matter) Jim Phelps(played by Peter Graves the rest of and the second series) as team boss.