Reading a book is not instantaneous like an explosion, or the blink of an eye, but an activity that requires some time. Nonetheless, when we use the past tense of the verb, as you do in your examples, it normally is understood to mean "to read a written work in its entirety" and not "to read page by page".
I read that book when I was in high school.
Therefore, when we wish to refer to a past act of reading in a manner that emphasizes the ongoing activity, sentence by sentence, page by page, chapter by chapter, we use the continuous:
While I was reading that book I found an article that interested me.
When we wish to refer to the activity of reading without emphasizing its ongoing physical/mental aspect and without the implication of completion, we can use the simple past in conjunction with a word like while which expresses duration:
While she knitted near the fireplace, her sister read a book near the window.
But when we use the simple past in that way, the idea of activity-happening-in-time is absent, or at the very least unexpressed. It is as though we were describing a moment frozen in time, a woodcut, a tableau vivant.