Every year since the beginning of the new century, the Serpentine Gallery in London has commissioned a summer pavilion from a famous architect. This summer, a new book Ten Years Serpentine Gallery Pavilions was published by Taschen Verlag to coincide with the opening of the eleventh annual pavilion, designed by Peter Zumthor. Helen Mallinson reviews Zumthor's pavilion here, in the context of the decade-old pavilion programme.