In the cold garden, they really should have left out more of the stones!
Incidentally, each of these haiku in The Essence of Modern Haiku includes some comments from the poet and some vocabulary explanations. I thought those pertaining to this poem were particularly interesting.
The stone garden of the Ryoan-ji, in Kyoto. In the cold garden, 15 stones are arranged in groups of 7, 5, and 3. Fifteen is the smallest number possible, but it seems to me, accustomed to working in the short poetic form of the haiku, that more stones should have been left out.
更に: “more, again.”
省く = “exclude, omit, eliminate”, and べし is a classical form implying “ought to, should”, so 省くべし literally means “ought to eliminate”. The intended meaning, though, is that more stones “could have been left out”, rather than that they should now be eliminated.