Not so. Certainly books on Waterloo will always sell, but as an author I have managed to get titles commissioned on some pretty obscure topics, and as a commissioning editor have signed up some that are even more obscure. Actually, one of the best bits of advice I would give is to find something - a campaign, a battle, a person, a regiment - that hasn't been studied before and to make it your specialism. 'The first book to cover...' is always a good sales pitch both for an author trying to get a publisher interested, and for a publisher trying to sell books.