Maisie Williams
Sophie Heawood of "The Guardian" described Arya's eight season character arc as a journey "from cute little sister to ruthless serial killer (of 64 victims)", and said that Arya was "the serial killer everyone's rooting for." Described as a fan favourite and one of the central protagonists in the Game of Thrones fantasy epic, Arya has a story arc across the first six seasons that encompasses severance, trauma, tragedy and revenge. A young character who witnesses both the unfair execution of her father and the immediate aftermath of her mother and brother's murder (including the parading of the latter's desecrated corpse). Regarded as an anti heroine, Arya works her way through a revenge death list of names of those responsible for killing all the people she cared about. After six seasons of surviving alone she reunites with her family as a weapons expert and trained assassin. When the expected heroes and armies of the living appear defeated in the longest battle scene in film and television history, Arya Stark saves the entire fictional continent of Westeros by using her prowess as an assassin to kill the Night King (the "god of death") which causes the simultaneous eradication of his vast undead army. Arya killing the Night King was nominated for the 2020 BAFTA TV Awards under the "Must-see moment" category. Williams, who did the majority of her own stunts and fight scenes in the series, is naturally right-handed, but kept in character by performing left handed in the show, and was told a year before the filming of The Long Night to build up her stamina for the episode. Williams appeared in all eight broadcast seasons of Game of Thrones, the final episode of which aired in May 2019. 
Maisie Williams
Williams received critical praise and recognition for her portrayal of Arya in the show. In her second year of professional acting, she was submitted in the category of Outstanding Supporting Actress for the 64th Primetime Emmy Awards by HBO, but did not make the nomination shortlist. Williams did win the 2012 Portal Award for Best Supporting Actress – Television (at 15 years, the youngest actress ever to achieve this) and the Portal Award for Best Young Actor. In November 2013, she received the BBC Radio 1 Teen Award for Best British Actor. August 2014 she won "Best Supporting Actress, Drama" in the EWwy Awards. In 2015, she was awarded the Empire Hero Award, and the Saturn Award for Best Performance by a Young Actor in a Television Series In 2016, she was nominated for a Primetime Emmy Award for Outstanding Supporting Actress in a Drama Series. In 2019, her performance in final season of the drama resulted in her again receiving the Saturn Award for Best Performance by a Younger Actor in a Television Series, as well as nominations for the Best Hero and Best Fight in the MTV Movie & TV Awards and People's Choice Awards for The Female TV Star and The Drama TV Star. Regarding Williams' second Primetime Emmy Award nomination for Outstanding Supporting Actress in a Drama Series in 2019, Daniel D'Addario from "Variety" said that Williams "entered the show as a child with minimal experience, but swiftly proved herself a very gifted performer... Millions watched her grow into her talents – and a fitting end to her very unusual journey through her first role" would be for her to win an Emmy.
Arya Stark
Arya Stark is a fictional character in American author George R. R. Martin's "A Song of Ice and Fire" epic fantasy novel series. She is a prominent point-of-view character in the novels with the third most viewpoint chapters, and is the only viewpoint character to have appeared in every published book of the series.

Question: Who was the author of the book series that the TV series is based on that Maisie Williams made her debut on?
Answer: George R. R. Martin