Brooks organizes them based on whether the authors would like each other:
Claire Messud and Alice Munro? I’m sure they’d get on. But Norman Mailer and Anne Michaels? I think not. Best move the poetic and exquisitely sensitive Michaels next to Andre Makine — a much better match. Mailer can slide back along the shelf to sit beside D.H. Lawrence. If nothing else, they can always brag to one another about their sex lives.
No Twain next to Austen I guess. But R.K. Narayan and V.S. Pritchett would be fast friends I bet.