Tankfield emailed Pea some photos, which Pea did not find interesting.
which就近指代photos，photos是定语从句中的宾语，把定语从句还原成一个完整句就是Pea did not find the photos interesting.
可以看到，上面两个句子都可以以that为关系代词改写，即
(1)Tankfield emailed Pea some photos that did not cheer Pea up.
(2)Tankfield emailed Pea some photos that Pea did not find interesting.
which前面是介词，介词前面是名词，which就近指代介词前的那个名词，名词和介词之间有无逗号均可。
Tankfield once lent Pea a book from which Pea glimpsed the basics of genes.
这个句子可以“拆”成如下两句：（每个定语从句都可以拆成两句，以先行词为拆分点）
(1)Tankfield once lent Pea a book.
(2)Pea glimpsed the basics of genes from the book.
先行词实际上是定语从句中介词的宾语，这时，需要把介词提前，避免如下表达：
(1)Tankfield once lent Pea a book, which Pea glimpsed the basics of genes from. (XXX)
(2)Tankfield once lent Pea a book that Pea glimpsed the basics of genes from. (XXX)
上中学的时候学过，which引导的定语从句叫“非限制性定语从句”，是不是？它怎么就“非限制性”了？回顾上述GMAT语法规定which有且仅有的两种用法，which指的都是就近的名词，从句其实还是限制性的。
所谓非限制性呢，是说which指逗号之前一套主谓所描述的情况，以逗号前的一个完整句为先行词。比如：
(1)Tankfield might never care to say Happy Birthday to Pea, which was rather sad.
(2)Tankfield might never care to say Happy Birthday to Pea, which Pea did not refuse to acknowledge to herself any more.