She glanced at Laura, a slim, dark-haired girl of fourteen, sitting beside her, and felt warm with that most comfortable of parental emotions, gratitude to one's own child. 

这里，that 就是 the 的意思，用指示代词代替the ，以加强语气。最高级 the most comfortable 具有名词特征，相当于名词，或者看作省略了跟后面重复的名词 emotion。而of of parental emotions 作其定语。整个that most comfortable of parental emotions 作with 的介宾。

又如：

When our class was assigned to Mr. Fleagle for third-year English I anticipated another cheerless year in that most tedious of subjects.

当我们班被安排给弗莱格尔先生教三年级英语时，我准备着在这个最单调乏味的科目中，又要度过不愉快的一年。

在in that most tedious of subjects中，that most tedious 是形容词的最高级，相当于名词或者省略了名词均可，这个名词就是of 后面的名词subjects。that 指示代词，相当于the。

▲ 形容词最高级具有名词功能，可以作主语、宾语或表语。例如：

Of all the lessons taught by the financial crisis, the most personal has been that Americans aren’t so good at money-management.

在金融危机给人们的所有教训中，最涉及个人的（教训）是美国人不那么擅长理财。

Courage is the most important of all virtues.  勇气是所有美德之首。

It was done with the best of intentions.  是出于一片好心才这样做的。

She is the most famous of all American movie queens.  她是全美最著名的影后。

After Taiwan, Hainan is the largest of all our islands.  除了台湾，海南是我国最大的海岛。