If you see the sign appears next to a book, that means the book is currently on sale at amazon.com.
Simply click the sign to read the reviews and the detailed information about the book.
When searching for books or the web, there may be more than one link for the same book in the search result. We ramdonly pick one as the "main" site, and mark the rest as [mirror] sites.
The benefit is: when the main site does not work well for you (for example, either too slow or broken), you can pick a [mirror] site if there is one.