To mark the 20th anniversary of the release of the first book in George R.R. Martin's Song of Fire and Ice series, A Game of Thrones, Penguin Random House has released an enhanced e-book edition of the title featuring additional content that is available exclusively via the iBooks Store.

A Game of Thrones: Enhanced Edition is intended to help readers learn more about the history of the characters in the series, and the "houses" that are featured in the book's world of Westeros. Enhanced editions of the other books in the series, title two through five, will be released over the next few months.

The enhanced editions will offer new cover art for the multi-volume work, annotations, character journeys and timelines and interactive character maps. The titles also include family trees, extensive illustrations, audio clips and a glossary.

Martin said the enhanced editions mark "a new period in the history of publishing." He went on: "Anything that confuses you, anything you want to know more about, it’s right there at your fingertips. It’s an amazing next step in the world of books."