The functionality had only appeared on Kindle readers and Kindle Fire tablets until today and gives readers a neat snapshot of a book's themes and characters and reveals where they pop up throughout the text.
Clicking on a character's name will bring up a timeline of their appearances throughout the book and in many cases a biography and a Wikipedia entry.
The supplementary information isn't accessed through the web, but is downloaded as an accompanying offline file when the book is purchased from the Kindle store.
The roll out for the iOS devices is an interesting one, as X-Ray is one of the few features that distinguishes the Kindle reading experience on Amazon's devices.
It shows, once again, that Amazon isn't necessarily concerned with which device users are reading their Kindle books from, as long as they're bought from its online portal.
If Amazon can make its reader experience better than Apple's built-in iBooks option then it can use Apple's own mobile devices against it.
Just last month Amazon rolled out the X-Ray feature for movies on Kindle Fire tablets, allowing users to access information from the Internet Movie Database (IMDb) when they pause the film.
Via The Next Web