Dan Rather says his upcoming memoir will set straight the facts of his firing from CBS. The news legend had his own tome on his mind at the party for Bill O’Shaughnessy‘s fifth book, “Vox Populi,” at Le Cirque on Wednesday. Rather confided to writer Gregory Speck, “Until now I never fully understood the power of the forces behind my axing from the anchor post at CBS News several years back, simply for speaking the truth about George Bush‘s questionable National Guard service.” Rather was fired in 2006 for claiming Bush had received preferential treatment in the Guard in the early ’70s using questionable memos as evidence. He tells Page Six he’ll set the record straight in his book due this spring, “Summing Up.” At the party, Rather also said, “With President Obama caving in to the Republi can demand for a tax freeload for billionaires, I can be lieve anything.” Whitney Radio president O’Shaugnessy was toasted by fellow authors including Gay Talese.