and to all this she must yet add something more substantial, in the improvement of her mind by extensive reading.
1. add...to...结构：add something more substantial to all this。为了突出状语以及因为宾语较长的缘故，将 to all this 提前。
2. in the improvement of ...介词短语，作目的状语。“为了改善/提高/增长...”。

类似表达很多，例如：He was studying hard,  in the hope of being admitted to an engineering college
他努力学习，希望能被一所工程学院录取。