Most people are aware that Amazon and Barnes & Noble both have ranking systems for the books they sell, but few understand what the ranking means. While rank has no bearing on how much any one person might or might not like a particular book, it does offer some insight into how well the book is selling.
The ranking number literally tells you where that book ranks among all the other books of the same format. The lower the ranking number, the better the book is selling, so a low rank is a good thing, and the #1 ranked book is currently the best-selling book. The rank is based on a secret formula known only to the bookseller, but generally, it is a combination of the number of books sold over time with a diminishing emphasis on older sales. The Amazon rank is updated about once per hour, and the Barnes & Noble rank, once per day.
Each individual version or edition of a book starts out fresh with its own sales rank, so a book that has been produced in three different paper versions (not just updates or fixes to an existing version) and is being sold concurrently in all those versions, will have three different rankings. Likewise, the ebook version of that book will also have its own ranking among all the other ebooks. For example, if you see an ebook ranked #100,000, that means it is the 100,000th best seller among all ebooks, not among all books.
To see how this works, consider The Girl with the Dragon Tatoo, by the late Stieg Larsson. This book has been a Kindle Top 100 bestseller for 785 weeks as of this writing. Dragon Tatoo is available as a Kindle ebook, a hardcover book, a mass market paperback, a high quality paperback, an audiobook CD, and an audiobook download. Each of these versions has a completely different ranking within its format group.
If you want some feel for what a particular rank means, consider how many total books there are of that type. Amazon currently has about 750,000 ebooks for sale, so an ebook with a rank of 75,000 is in the top 10% of ebook sales at Amazon. Barnes & Noble has closer to two million ebooks, so a rank of 75,000 there would put the book in its top 3.75%. On the other hand, Amazon probably outsells Barnes & Noble overall in ebooks, so the top 10% book on Amazon is probably selling significantly more copies than a top 10% book on B&N.
If a book has no rank listed at all, then it has never sold a copy through that vendor, in the current edition/version, even if it was a top seller in some other format.
The important point to keep in mind about book rankings is that they are only a measure of sales, not a measure of quality. Just as a lot of people will give two-thumbs down to huge box office blockbuster movies, not everyone will enjoy a book that's a consistent top ranker at the online bookstores. For example, if you look at the reviews for The Girl with the Dragon Tatoo, you'll see that while half the readers gave it five stars, about a quarter of the people who read and rated the book only gave it one or two stars. Thus, even a book that is enormously popular based on sales numbers can turn out to be a dud for many readers, or more simply, beauty is still in the eye of the beholder.