I don't really know too much about the author Kitty Kelley or her reputation. Nor do I know that much about the reliability of the British paper The Mirror. So, I'll leave it up to you to judge the validity of either as a relevent source. With that preample out of the way, here is a piece from The Mirror about the upcoming Kitty Kelley book, "The Family : The Real Story of the Bush Dynasty":
"Author Kitty Kelley says in her biography The Family: The Real Story of the Bush Dynasty, that the US President first used coke at university in the mid-1960s.
"She quotes his former sister-in-law Sharon Bush who claims: 'Bush did coke at Camp David when his father was President, and not just once either.'"