The Amazon Kindle and Fire tablets have a feature called X-Ray. It gives you a rundown of the people, places and things in a digital book that is purchased from Amazon. If you are reading autobiographies with lots of images, there is also an X-Ray function to look at all of the images in the book. One of the things that most readers find compelling, are all of the characters in the book. X-Ray breaks down all of the major and minor characters, how many times they have been referenced and you can jump immediately to the page of the book. This not only works with characters, but also places and images.
X-Ray is a reference tool, that was first introduced in September 2011, and was incorporated in the Amazon Kindle Touch and later models, Kindle Fire tablets, Kindle apps for mobile platforms, Fire Phones, and Fire TVs. General reference information is preloaded into a small file on the Kindle device or app, so that when the feature is used, there is no need to access the Internet to access such content as dictionary, encyclopedic, or metadata entries.
Normally X-Ray is enabled on all books that Amazon sells in their digital bookstore and you can check ahead of time on the Amazon site when you’re browsing books for those that support X-Ray. On the detail page for a book, scroll down to the Product Details section. Next to X-Ray, look for Enabled.
There are a few caveats with X-Ray, they do not work with sideloaded books. This includes books you copy over with a file manager or 3rd party apps such as Calibre. Send to Kindle for Chrome, PC, MAC and Kindle apps for Android and iOS also do not work with X-Ray.
One of the biggest strengths of X-Ray is gaining a greater understanding of complex books. It also is tremendously relevant when reading a series with a ton of different books and make references to hundreds of names and places. If you take a break from reading and jump back to it a couple of months later, it is easy to get lost. I found this to be a lifesaver with the Wheel of Time series or Brandon Sandersons Stormlight.