How easy is it to get out of a book contract?

How easy is it to get out of a book contract?

Derek Johns (Managing Director - AP Watt Literary Agency) gives expert video advice on: How long does it take to go from finding an agent to getting published?; What happens when a book is accepted for publication?; What should I do if a publisher asks me to make revisions before they accept my submission? and more...

Once an author has signed a book contract, then like any contract, it's binding. But if things simply break down or the author can't write the book, then by simply repaying the advance an author can get out of a contract.