Well, I never knew such a book existed, but Robert T. Carroll reviewed it back in Oct 22 1996: http://www.skepdic.com/refuge/hubbard.html

Book Review
The Sound of One Mouth Blathering
review of

The Rediscovery of the Human Soul
by L. Ron Hubbard
Scientology, 1996

Robert T. Carroll
October 22, 1996

Which of the following does not belong? The Talmud, the Upanishads, the New Testament, the Vedas, Dianetics?

Here is an even easier question: which of the following does not belong? Judaism, Islam, Taoism, Buddhism, Scientology?

Despite what its founder and its advocates say, Scientology is not a religion. It has no creed, no rituals, and no hope of becoming a major social institution for the transmission of values. It has no cosmological myths and offers nothing new or interesting in ethical teachings. What it does have is philosophical dogma which it claims is scientifically validated by its practice of auditing. And while these dogmas do assert belief in a soul which is independent of the body and which usually resides in a person's head, the origin of the soul is obscure while its destiny is vaguely described in Buddhistic terms of escape from the cycle of rebirth. Scientology is an eclectic collage of philosophical and religious notions imaginatively brought together in a loose system by a man with a gift for fantasy.
I received an unsolicited copy of The Rediscovery of the Human Soul. It was sent to my office in the Philosophy Department at Sacramento City College. I also received an invitation to think about it from Kaye Copley, the L. Ron Hubbard Public Relations Director. Copley writes that the book was sent to me to "provide an understanding of some of the fundamental principles of Scientology and an insight into what lies behind its phenomenal growth." And, "We look forward to receiving your thoughts on [Scientology]."
Well, here goes.

Slick but unimpressive

I'll begin with the positive. This book is beautifully done: slick, glossy pages, colorful photos, artistic black and white shots and graphics. It's pretty enough to go on the coffee table.

Now to the content and the negative.

The book is a collection of articles either by or about Hubbard. In "An Introduction to L. Ron Hubbard" we're told that no "single philosophic work" is as large as the materials of Scientology. This is unimpressive. Bigger is not necessarily better, or even good, for that matter.
It is a fun review - one I think you'll enjoy such comments as this one:
Scientology might best be described as Irrefutable Pragmatism. The only criteria for truth, we're told, is "workability." Did procedures "better our capacity to survive, actually make us happier, more causative and more able?" The beauty of such imprecise concepts is in their slipperiness: there is no need to invent ad hoc hypotheses to explain disconfirming evidence because the concepts are so vague that one can fit any data to them. I fear that the "axioms" of Scientology which are not false or trivial are likely to be so vague as to be useless.
Check it out - it's worth the read.