Book excerpt: Mark Forsyth - author of the Sunday Times Number One bestseller The Etymologicon - reveals in this essay, specially commissioned for Independent Booksellers Week, the most valuable thing about a really good bookshop. Along the way he considers the wisdom of Donald Rumsfeld, naughty French photographs, why Elizabeth Bennet and Mr Darcy would never have met online, and why only a bookshop can give you that precious thing - what you never knew you were looking for.