In particular, she claims that "extreme" fundraising activities are being "driven from within the very highest echelons of the Scientology structure" in a way that is at odds with the organisation's founding scriptures.
Although many current members have donated vast portions of their net worth to the church, Cook claims that Hubbard never endorsed individual donations of more than US$75 for lifetime membership.
The church's founder also "never authorised the purchase of opulent buildings", she says. Instead, he believed that all money raised by the organisation should immediately be used to spread its message.
Now aged 50 and living in Texas, Cook was for three decades a member of "Sea Org", Scientology's equivalent of the clergy. Between 1999 and 2006, she was Sea Org's top representative at the church's world headquarters in Clearwater, Florida.
Later, she moved up to its ranks of management, and between 2006 and 2008 was at its offices in California.
It was in that capacity that Cook directly witnessed Miscavige's management style. She now claims it subverts the checks set up by Hubbard to prevent any one individual taking control of the church.
In 2008, she left the church's payroll to start an internet firm with her husband. But she remained an active Scientologist and says she remains "completely dedicated" to its beliefs.
Following her email, several former "friends" used Cook's Facebook page to announce that they were "reporting" her to Scientology's "ethics" department.
A church spokesman said: "Ms Cook's opinions reflect a small, ignorant and unenlightened view of the world today. They are not shared by thousands of Scientologists who are overjoyed by our 27 new churches and what they mean to the communities they serve."
Intergalactic church of the rich and the famous
* The Church of Scientology was set up in the United States in 1954, and claims to have eight million members worldwide.
* It offers self-improvement on the basis of the writings of the late science-fiction author L. Ron Hubbard, who spelled out principles that he called Scientology and Dianetics.
* Scientologists believe that humans are immortal beings who have forgotten their true nature, and should embark on a course of "spiritual rehabilitation".
* They also believe that an intergalactic despot once brought billions of prisoners to earth in a fleet of spacecraft, and that the key teachings are not revealed until the faithful have paid thousands of dollars to the church.
* Famous Scientologists include actors Tom Cruise, John Travolta, Giovanni Ribisi and Jason Lee and musician Beck.
What Debbie Cook said
David Miscavige has now become the 'leader' of the Scientology religion ... There never was supposed to be a 'leader' other than [L. Ron Hubbard] himself as the goal maker for our group.
There is no question that this new age of continuous fundraising is not our finest moment.
I am sorry that I am the one telling you, but a new storm is upon us. Its waves are already in the media and the world around us.
- Independent