No doubt it's a lot, nay, all to do with Forbes no. 1 celebrity Oprah Winfrey, as it was Oprah that featured Kathy Freston's book, the Veganist on her show, and on the Oprah website, and got 378 of her staff to follow the book's advice for a week.
The American Vegan Cookbooks dominate sales of vegan recipe books, (especially ones about cupcakes) even on amazon.co.uk you won't see a British vegan cookbook tackling vegan diets until number 6 on the 'Vegan cookbooks' listings.
Its a good book too even if I do say so myself, so many of the books don't have pictures and us cooks like pictures. The Complete Book of Vegan Cooking is full of them, but is currently down to rank number 41 in the Vegetarian and vegan section, despite selling out and having to be reprinted within 6 months of it's publication.
Still, having a vegan cookbook in the bestsellers list bodes very well for the Plantarian 2011 One World Day.