The Amazon Search Inside the Book program is a free program that provides a preview of your book on the Amazon site that is designed to entice buyers to purchase your book by letting them browse it as they would in a physical bookstore. The pages of the book appear with watermarks on them to protect your content.
Do I need it?
It is not essentially but, for any author, this program is a big marketing benefit and is well worth the effort of joining. In addition to the fact that it is a powerful marketing tool as it allows buyers to preview your content before they decide to purchase, it is an important tool for SEO (Search Engine Optimization). Both Amazon’s internal search engine and search engines like Google, Yahoo, etc pull not only from the listing for the book (title, description, etc) but also the content. Thus buyers are more likely to find your book when searching for anything mentioned in your book even if you did not specifically mention that topic in the book’s meta-details. It increases the chances of potential buyers finding your book and then, when they get to preview before buying, increases the chances that they will purchase.
How do I join it?
Publishers or Authors can both sign up for the program here: http://www.amazon.com/gp/help/customer/display.html?ie=UTF8&nodeId=14061791
Once you sign up for the program, Amazon will give you instructions for submitting your book. Smaller publishers may be asked to mail in a paper copy of their book for scanning while some publishers will be given the option to upload the electronic file of their book for digital processing. While physical processing must surely be slower and more time/money intensive than electronic processing, Amazon has yet to move to an all digital submission process.
If you are a member of the Amazon Advantage program, the option to submit your books to the Search Inside the Book program digitally is already available in your account so there is no need to sign up again.
Definitely a big benefit as you say. Unless I know exactly what I’m buying from Amazon that (rare!), I always look inside the book for more info and more times than not it’s the contents page that gets the sale. With skepticism rising over the validity of Amazon customer reviews, there’s no better way to confirm the title is what you want than looking inside.