Books of like feather often seem to be sold together. Sometimes the reasons for this are obvious. In January 2006 I found myself selling every copy of a compilation of 'Peter Simple' columns that I had. I then realised that the creator, journalist Michael Wharton, had just died. A writer's death often increases the interest in their work so that is not that surprising. Neither was selling out of copies of Gowk Storm by Nancy Brysson Morrison (excellent Canongate edition introduced by Edwin Morgan) after a particularly good radio adaptation, as such serialisation is bound to bring interest.
What is more odd are the days when I seem to sell on a theme. On one occasion every book sold that day had Marx (or Marxist) in the title, and that kind of thing seems to happen more often than you'd think. The oddest was selling, over the course of 24 hours, three different books on different subjects that all had part of the same painting on the cover: "Coalbrookdale by Night", by Philip de Loutherbourg. You can see it here on one of those books, The Romantics: England in a Revolutionary Age by E. P. Thompson.
Another funny one was selling a book to my childhood home village and a few moments later another book to my husband's childhood home village. Neither village is near here, or near each other.
I suppose a mathematician would say that I am misunderstanding coincidence; that books are complex objects, with so many facets (contents, author, themes, illustrators, design, publisher, country of origin, etc), and the customers too, have lots of features, and that in some of these large lists coincidence is more likely than not amongst the many features. And I guess that's true.
So it is very much a moment of folk wisdom when I look at a day's sales and think, 'How odd!' But it happens a lot.