Leading authors Haruki Murakami and Martin Amis will also make their debut appearances at Edinburgh International Book Festival.
This year's event, which runs from August 9 to 25, has the theme Let's Talk and will include a series of evening debates about Scottish independence and the looming referendum poll.
Festival director Nick Barley admitted he could have sold out the main theatre events many times over for George RR Martin - author of A Song of Ice and Fire on which the TV series Game of Thrones was based, and Diana Gabaldon, who wrote the Outlander series.
He said: "They are not both regular festival-goers and it is an honour they are attending.
"George RR Martin could have sold out our theatre many times, but the same could be said for 30 or 40 authors in the programme this year -what's crucial to me is that these events are not about packing in the most amount of people, but creating a dialogue between writers and readers."
Mr Barley said the festival would be a "neutral ground" for the independence debate.
He said that writers and thinkers involved in the Scotland's Future strand had not been asked whether they would be voting Yes or No in September.
Mr Barley said dialogue of all kinds, not the referendum itself, was central to the festival but acknowledged this year's could be particularly politically charged.
Those involved in referendum-related events include Herald columnists Iain Macwhirter and David Torrance, historian Linda Colley, James Robertson, Lesley Riddoch, Professor Tom Devine, Gerry Hassan and former First Minister Henry McLeish.
Neither Alistair Darling, who is heading the Better Together campaign, nor First Minister Alex Salmond, both former guests at the festival, feature on the bill.
Mr Barley, who declined to comment on JK Rowling's support for the No campaign, said: "We have speakers from across the spectrum, and our job is to reflect the world of words - we have not operated a quota system.
"We haven't started by asking 'are you Yes or are you No?' because frankly that's not the focus of our endeavour, and in any case many people have not pronounced their position or decided
"What we're trying to do is reflect the written contributions to the world this year ... and I am absolutely confident that our neutral position will allow every view to be heard, and also it is perfectly possible that people will arrive at an event with one view and leave with another."
He added that flyering and leaflet-distributing was not allowed at the festival site.
Haruki Murakami will be in Edinburgh for the first time, and is launching his new book, Colorless Tsukuru Tazaki and His Years of Pilgrimage, which sold one million copies in Japan in its first week of publication.
Martin Amis will be launching his new novel, The Zone of Interest, and former Archbishop of Canterbury Dr Rowan Williams will make two appearances, presenting his new poetry collection The Other Mountain and discussing the idea and impact of religious belief.
Tickets for the festival go on sale on June 24.