I love talking about metadata. It's the most important tool you have for selling your book online today, yet probably one of the least well-understood.
Metadata is data about the data. It's like the view of a city from above. Book metadata comprises all the little bits of information that are collected by the publisher and shared online with retailers via electronic feeds.
Get the metadata right and your book will be easily found by the exact right audience. Get it wrong, and it's possible a good book will be lost.
The most obvious pieces of this information include the book's title, subtitle, author's name, author's bio, book description, BISAC (subject) code, ISBN, publication date, imprint, and even professional book reviews. Filing the book in the correct subject category and including keywords for important geographical locations, time in history, and topics covered in the book (i.e. father-daughter relationships, whaling ships, scrimshaw, Scandinavian history) make all the difference as to whether readers searching for a good Icelandic Viking mystery will find your book or not.
A book's cover art and the back cover copy are also metadata. If the cover and the copy don't objectively match the book's genre or the content, it's not telling the right story. It's worth fighting to get this part right because readers do make snap purchase decisions on covers and descriptions they see in bookstores and on the web.
If your book has been through significant revisions in the editorial process, or if the cover changed at some point, it's important to check your book's information online. The metadata of the book may have been filled out by an assistant early in the process and never updated, and old covers and descriptions may have been sent out with the catalog copy.
Sometimes the metadata might be flat-out wrong. I've seen online bookstore listings that include the descriptions of different books from the title and cover that are displayed.
So, check that your listing is correct. Metadata about the book can be updated nightly by the publisher's systems or via your self-publishing dashboard. The best place to start is to check your Amazon and Bookwire.com listings.