请教一个新概念英语课文（39课）的一句英文语法

请教一个新概念英语课文（39课）的一句英文语法
I have known very few writers, but those I have known, and whom I respect, confess at once that they have little idea where they are going when they first set pen to paper.
其中，those I have known 是否为those whom I have known 的省略，为什么 and whom I respect 要加上whom,不能省略吗？