I've tried reading outside in all seasons, including winter, and I've concluded that spring is the best season for outdoor reading. It's not too hot and it's not too cold, the bugs aren't out yet, and when the rain walls down, the porch becomes the ideal reading room. The only distraction: The greening and flowering of the earth, combined with the play and shouts of children just released from the bottle of winter.
But those things are better than books.