When Mobil Oil's vice president for public affairs, Herbert Schmertz, joined Sen. Edward Kennedy's campaign staff last month, many corporate and political types were perplexed.What, some asked, was big oil's smoothest spokesman doing working for a senator who fought for decontrol of oil prices? And even though Schmertz had worked for Jack Kennedy's administration and Bobby Kennedy's presidential campaign, others wondered why the Senate's foremost liberal hired the voice of Mobil Oil to plot his media strategy.

Schmertz isn't saying much to the press, but next month his corporate friends may be surprised again when Simon & Schuster publishes a novel he wrote with another Mobil vice president, Larry Woods. In the book, Takeover, the bad guys are none other than business executives.

The commercial ingredients are there for a potboiler: sex and dirty dealings by execs who find a way into the computer at the Department of Housing and Urban Development and divert federal funds into a dummy account. In the end, though, the bad guys become good guys and quietly return the loot with interest.

"With the special knowledge that only true insiders can provide," the proposed jacket cover reads, "the authors lead us behind the locked doors of the boardrooms and bedrooms of the rich and powerful to witness one of the biggest and most sensational business conspiracies in fictional history."

To his friends, Schmertz is a cool enigma. His bid to become the Harold Robbins of the oil industry may not shed much light on the surprise detour in his corporate career, but it should be great reading for those who have been with him behind locked boardroom doors.