The SNP has yet to convincingly map out a future for Scotland, the director of the nations's biggest book festival has declared on the eve of this year's event.

Nick Barley, director of the Edinburgh International Book Festival, which opens tomorrow, said the festival's theme, 'Imagine Better', applies as much to the ruling SNP administration as any other topic to be discussed at the annual fortnight of literary, political, philosophical and lyrical discussion.

He says the SNP need to "map out the future for Scotland" beyond being a "useful managerialist" government.

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Mr Barley made the comments as the festival unveiled details of three major new discussions to be staged at the festival: on Scotland, on Europe, and on the future of NATO.

Nicola Sturgeon, the First Minister, is appearing at the book festival, in a discussion with the Makar, Jackie Kay.

Mr Barley said: "The time has come now, I think, for this government, and for Nicola Sturgeon, to map out the future for Scotland, not just in terms of independence or devolution, but in terms of what this country can be, a long term vision for the future of Scotland.

"There are those people, this is not my view, but there are those who would say that the SNP has yet to fulfill its potential, given its been so popular and given it's been so successful: where is the success story? What can they talk about?"

He added: "If their reason for being is independence, then it follows that a second referendum should be on the cards.

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"If the second referendum is being put back, being kicked into the long grass by a set of criteria which Nicola Sturgeon cannily knows are difficult to fulfil, then the question is: what else? What is this government doing?

"And a useful managerialist regime is great, but that won't be enough for the people who voted SNP again and again in recent years and are expecting more.

"So the phrase 'Imagine Better', the title of this year's festival, is intended also to remind politicians, journalists, novelists, writers, economists, scientists and everyone at the festival that we have to articulate what we imagine, and share it with people, if we are to achieve something better."

The book festival has released details of three debates which will discuss contemporary political affairs.

On Monday 15 August, chaired by David Torrance, the Scotland Now debate will feature Alex Bell, Aileen McHarg and Alex Massie.

The debate, The End of the Trans-Atlantic Era?, chaired by Allan Little, will discuss NATO and the possible consequences of a Donald Trump presidency, with Menzies Campbell, the novelist Lionel Shriver and Steven Erlanger, the London bureau chief of the New York Times, will take place on August 18.

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A debate about Europe and Brexit will take place on August 29, chaired by Charlie Jeffrey, and will feature Alyn Smith MEP and Giles Merritt.

Mr Barley added: "It's pretty likely that Brexit and the effects of Brexit are going to be a big topic of conversation.

"We did programme in mind with two major political events in mind, the Scottish election, and the European referendum, and so many of the debates with the idea that this might be an issue, either way.

"We did have an idea that [former Prime Minister] David Cameron was gambling the house, even with a narrow victory his position might have been difficult, so we set up an event about the legacy of Cameron, knowing it was quite possibly be the final story [of his Prime Ministerial career]."

The festival, which runs until August 29, is once again in Charlotte Square, Edinburgh.

It features more than 800 writers, authors, thinkers and poets in 750 events.

Barley added: "This forum is a very good place for thinking of somewhat Utopian ideas, but actually how are to make this country better for our children? Book festivals are a good place to do that.

"We are operating in a media world which is dominated by soundbites, 140 characters [on Twitter], clickbait, headline culture, that is potentially detrimental to the quality of our discourse," he said,

"Many people felt a bit sullied by the [European] referendum discussions, partly because it came down to a binary decision, yes or no, and all of the subtleties of what yes or no mean.

"It was particularly stark in the EU referendum because the quality of discussion was poor as well.

"A book festival is where those binary discussions aren't so important, its a place where we can have complex discussions, and a necessary corrective to the clickbait culture."

Mr Barley also noted that he would be one of the judges of the Man Booker International Book Prize next year.