THE UNMAKING of Premier David Peterson in 1990 liberated Georgette Gagnon and Dan Rath from the Ontario payroll and left them leisure and a publisher`s advance to find out why their careers had been derailed. Their answer, delivered in Not Without Cause: David Peterson`s Fall from Grace, a Cautionary Tale for Voters and Politicians verywhere (HarperCollins, 408 pages, $22.95 cloth), is pretty much what everyone else had figured out a year ago. Re-elected in 1987 because the NDP Liberal accord made him popular, Peterson was promised an encore by poll sters only three years into his term. In fact the Liberals had sleepwalked since 1987, doing little, favouring rich friends and alienating single-issue groups, nice and nasty. Peterson`s valiant efforts to save the Meech Lake Accord helped turn the electorate sour. But heck, who cared? At 50 per cent in the polls, there were votes to spare. It turned out that Liberal pollsters were wrong and Ontario voters cared. Instead of re-electing a party too laid back to run a decent campaign, much less Ontario, they gave the other party to the 1987 accord a chance. Almost resigned to defeat and retirement at age 42, Bob Rae was the astonished winner. Inevitably superficial but needlessly overweight, Not Without Cause will entertain and annoy Liberals still waiting for Carl. cheques. Their NDP successors, busy repeating some Peterson-era blunders, might well learn more from it. They are probably too busy.