Catherine Lockerbie has been in the post for nine years
The Edinburgh International Book Festival director is to step down from the post after a long period of illness.
Catherine Lockerbie's contract finishes on 31 August but she has been on sick leave since March.
Colleagues have praised her "nine highly successful" years in the role.
Richard Holloway, chairman of the Scottish Arts Council, is 2009 guest director of the festival. Ms Lockerbie conceived this year's programme.
She plans to move on to an international consultancy role in culture and literature.
Ms Lockerbie said: "It has been an unbelievable privilege to lead, nurture and build a festival dedicated to the power of words, ideas and challenging discussion.
"I am incredibly proud of our world-class team and what we have all achieved.
"The book festival has been a personal mission of mine, and I have poured every ounce of my energy, passion and thought into what has been an all-consuming role.
"Following a period of medical leave, I believe this to be a propitious moment to move on to new challenges and opportunities."
Poet Laureate Carol Ann Duffy said: "Catherine Lockerbie has established the Edinburgh International Book Festival as the finest of its kind in the world.
"Her achievement shines like a beacon throughout the global world of literature.
"She has been a literary director of integrity, vision, compassion and inspiration and writers and readers everywhere are forever in her debt.
"She leaves the post with the deep love and gratitude of every man, woman and child who ever attended her festivals."
Ms Lockerbie, whose work has been recognised by five honorary doctorates, among many other accolades, became director in 2000.
The Edinburgh International Book Festival has become the largest and most pre-eminent event of its kind in the world, with 200,000 people attending more than 750 events each year, and with authors, both world-renowned and new, invited from more than 45 different nations.
Susan Rice, chairwoman of the Edinburgh International Book Festival, said: "Catherine has taken the book festival into the major leagues, expanding the size and quality of its programme, the number of visitors and the praise it earns in the public eye.
"She has even seemingly expanded Charlotte Square Gardens because, though no one thought we could fit any more in, she has managed to do that as well.
"Catherine moves on from an outstanding term as director of the book festival.
"She leaves us in good heart and in good hands and she goes with our gratitude, our pride in what she has achieved and our love."
It is hoped a new director will take up the role in time to shape the 2010 programme.