The Secret* started out as a DVD* and is now available as a book* and audio book*. The secret that it's about is the Law of Attraction. Referring to it as a secret that has been kept from the masses is a clever marketing ploy. In reality, I think it's really a secret that has just been ignored by the masses.
The format of the DVD* and the book* is quite original. It's a mixture of quotes, interviews and documentary. It works really well as a way of explaining what is effectively a very simple concept that could be summed up in a few words. It's not just filler material though, it's well structured, well presented and well justified by using popular modern day self improvement gurus to support it.
However, as a very scientifically minded person, I found the pseudoscience (and bullshit) that was used to "prove" that it works was insulting and only served to discredit the creators and the Law of Attraction itself. There are scientific explanations for how and why it works, but quantum mechanics has nothing to do with it. By claiming that quantum mechanics proves that the universe works by human thought is moronic and shows no understanding of science or the scientific method.
However, I realise that The Secret* is aimed at people who are not scientifically educated, so this bit of mumbo jumbo probably serves to strengthen the principle for those people.
For people that are unaware of the Law of Attraction, The Secret* is a very good introduction to the principle. For those that are aware of the principle, it serves as a very good way to strengthen it in your mind and should hopefully convince you to start using it, or at least to try it.
For those that are very aware of the Law of Attraction and are already using it, it will be of limited value to you and would only really be useful as a matter of interest.
The book* is largely just a transcription of what's on the DVD*, so choose the method that you prefer. The DVD* gets everything across a lot quicker than the book*, but the book is a lot easier to use as a reference material afterwards.
So, in summary, I like it. It's a great introduction and a nice refresher, but ignore the pseudoscientific explanations.
