What does Dianetics have to do with scientology?

I know that it is a book/ program that is supposed to help people who are mentally ill, but i don't know how it links/ is linked to Scientology?

it is holy book of Scientology, written by its founder Ron Habbard

The most obvious link is that they are the brainchild of that supreme huckster: L. Ron Hubbard.

http://www.allaboutreligion.org/church-of-scientology.htm

Church of Scientology: What Is It?
The Church of Scientology International was founded by L. Ron Hubbard in 1954. Based principally on his book, Dianetics: The Modern Science of Mental Health (1950), Hubbard created a ?religious? institution to promote his unique psychological theories. Organizations associated with Scientology include Applied Scholastics, Association for Better Living and Education (ABLE), Citizens Commission on Human Rights, Concerned Businessmen of America, Hubbard Dianetics Centers, Narconon/Criminon, Religious Technology Center, Sterling Management Systems, and The Way to Happiness Foundation. Publishing ventures include New Era Publications, Bridge Publications, Inc., and Freedom magazine.

hutch of Scientology: Who Cares?
The Church of Scientology has been called a "thriving cult of greed and power. . .a ruthless global scam. . ." (Time magazine, May 1991). It has also been declared a "dangerous cult. . .a racket disguised as a religion. . ." (Reader's Digest, October 1991).

Scientology always seems find its way into the popular media. If it?s not a Miltie-million-dollar lawsuit, it?s a famous personality making a controversial statement. All we know is that Hubbard was an absolute genius. He was a fantastic speaker and prolific writer who sold millions of books and made millions of dollars by creating and marketing his own religion. In 1949, L. Ron Hubbard was quoted as saying, "If a man wants to make a million dollars, the best way would be to start his own religion." Well, it seems that Mr. Hubbard proved himself right.

The word Dianetics comes from the Greek words "DOA", meaning through and "nous", meaning soul. The full and proper definition of Dianetics is what the soul is doing to the body through the mind.

Dianetics is a book written by L. Ron Hubbard who is the founder of Scientology - his own self-made religion. This book was NOT written to help people who are mentally ill! Please check your facts before saying ignorant things. The link about Dianetics and Scientology is listed below.