'One source tells of an occasion when the Prince asked his famous occasional adviser to read over a speech he was due to give on a topic unrelated to health care or any field in which Savile had expertise,' wrote Mayer. She said Savile did not make any changes on that occasion.
The book sheds new light on the extent to which the Prince relied on Savile as his confidant.
Savile died in 2011 and was exposed as a prolific paedophile, using his celebrity status to prey on child victims. He is said to have abused hundreds of victims, taking advantage of having free run of Stoke Mandeville Hospital, Broadmoor and Leeds General Infirmary.
The book also deals with Prince Charles' relationships, revealing that he had doubts about his wedding to Princess Diana and on the eve of the ceremony said 'I can't do it.'
Sarah Goodall, a former Lady Clerk to the Prince previously claimed in an autobiography that Savile gave marriage guidance counselling to the Prince and Princess of Wales. This claim was denied by Clarence House.
The biography also says the Prince no longer wants to promote the interests of British arms firms in the Middle East.
Prince Charles, who like other members of the Royal Family has close links to many ruling families in the Arab world, particularly Saudi Arabia, told ministers that he 'doesn't like being used to market weaponry'.
Last year BAE Systems announced a deal to sell Typhoon jets to the Saudis the day after the Prince had finished a visit to the country.
Although aides insisted the deal was 'never discussed', anti-arms campaigners said it was 'clear that Prince Charles had been used by the UK Government and BAE Systems as an arms dealer'. The book claims his objections were made 'discreetly.'
The prince will visit the country for two days next week - for the 12th time - as part of a whirlwind tour of Arabia, also taking in Jordan, Kuwait, Qatar and the UAE.
Mayer wrote: 'A source close to the Prince says he doesn't like being used to market weaponry and now sidesteps such activities wherever possible. '