She will talk about her life outside of politics, including television appearances on Strictly Come Dancing and Doctor Who.
Former Corries musician Ronnie Browne, along with authors Christopher Brookmyre and Lesley Riddoch, will also be festival guests.
The 12th annual event will be held from 1-6 September.

The Dow Jones was up 16.5 points to 18,533, the S&P 500 gained 5.15 points to 2,166.8, while the Nasdaq rose 26.1 to 5055.7 points.
Bank of America shares rose 3.3% despite a 19% drop in quarterly profits, which still beat expectations.
Shares of other US banks also rose, with Citigroup climbing 0.5% and Wells Fargo up 1.2%.
Technology stocks also rose on Monday.
Apple shares gained 1.1%, Google's parent company Alphabet rose 2.4% and Facebook was up 2.2%.
Streaming service Netflix climbed 0.4%, but plunged 16% in after-hours trading after missing subscriber targets in the second quarter following price rises.
Yahoo shares added 0.6% ahead of second quarter results.
Former Conservative politician Ann Widdecombe will headline this year's Nairn Book and Arts Festival.
[NEXT_CONCEPT]
(Close): US markets reached fresh intra-day records, lifted by technology and banking stocks.