As many of us have experienced, reading assigned books is part of curriculum, sometimes these assigned books are difficult to read as they were written in another decade or another century. Unfortunately for us, the English changes, and it changes often, some words that were used with a certain context in the 1980s would be used in a completely different context today. As we are used to the English language as it is today, it is difficult for us to understand the English language of times past without effort. Often times, our brains would completely blank out and we would completely be unaware of what we are reading while still reading. This phenomenon is explained in this quite interesting Reddit thread.
From reading the Reddit topic and my own personal experience I found that visualizing helps to understand books that are difficult to understand, visualization also helps me with remembering key events in the plot. After reading a part that I do not understand completely, I reread the section and try to visualize the book in my head. Another method I found that works is discussion, often times different people discover different connections and interpretations while reading the same book. In discussion you can share these connections and interpretations to create a better understanding of the book. That being said, the Tale of Two Cities is a difficult book to understand thoroughly for me and by incorporating these techniques I have gained a better understanding than if I were to just read it normally.