As Game of Thrones prepares to wrap its final season on HBO next year with its untitled prequel series expected to begin principal photography sometime at the beginning of 2019, author George R.R. Martin has admitted the sixth installment of his A Song of Fire and Ice novels, from which the television series are based off of, has been difficult to write.

The acclaimed novelist has been writing The Winds of Winter, the penultimate novel in the popular dark fantasy series since the publication of the fifth book, A Dance with Dragons, in 2011. Originally anticipating completion in approximately three years, it has taken Martin seven years with no announced end in sight.

RELATED: George R.R. Martin Developing Even More Shows For HBO

"I’ve been struggling with it for a few years," explained Martin in an interview with The Guardian. "The Winds of Winter is not so much a novel as a dozen novels, each with a different protagonist, each having a different cast of supporting players, antagonists, allies, and lovers around them, and all of these weaving together against the march of time in an extremely complex fashion. So it’s very, very challenging. Fire & Blood, by contrast, was very simple. Not that it’s easy – it still took me years to put together – but it is easier."

Fire & Blood is the first half of the House Targaryen family history written by Martin and scheduled for publication on November 22. Despite taking time to write the history and developing other projects for HBO, including co-writing the pilot for the Game of Thrones prequel series, Martin assures readers that completion of The Winds of Winter is his next priority.

RELATED: Game of Thrones Spinoff Still Officially Untitled, George RR Martin Clarifies

There is currently no announced release date for The Winds of Winter.