Start with these…

Video: Tory Christman speaks at CFI – an ex-Scientologist shares her experiences. This set of videos is a great starting point in finding out why so many people dislike Scientology.
(Tory’s presentation start at about 8 minutes in to the first video on that page. See also her personal site.)

Thriving Cult Of Greed And Power – the highly-regarded 1991 Time Magazine article about Scientology.

We Are Legion: Anonymous and the War On Scientology – a good background in how the current wave of protest-activity got started, by Jeff Jacobsen, a long-time critic of Scientology.

Further reading

Audio: John Duignan radio interview in Dublin, about how Scientology lures vulnerable people in with false promises. (John Duignan recently released a book about his experiences in Scientology. It has no UK publisher, but Eason will happily post it to you from Ireland.)

Video: Scientology Victims Testimonies – personal accounts of many people who have left Scientology.

Video: Ron deWolf – L Ron Hubbard’s son – gave a damning TV interview about his father and Scientology.

Ex-Scientology Kids – more individual histories, this time from children who were brought up in Scientology.

Xenu.net – this site is the absolute motherlode of detailled information. If you want a huuuuge amount of stuff to read, this is the site.