Cowgirls, Hockney, and how to write a bestseller

With Stig Abell and Thea Lenarduzzi – Gerri Kimber on the role of women in the rise of the Western (plus the notorious case of Mrs Clem); as Tate Britain unveils the most extensive David Hockney retrospective yet, one of the show's curators talks us though some key moments, and themes, in a long and eclectic career; what makes a bestseller? Daisy Hildyard considers four new books that purport to tell us why some books succeed while others flop.