読解 and 読書: Understanding a passage and reading books
Compare interpreting a passage with spending time reading books. Keep the comprehension task, help from historical context, and reading duration separate from claims of complete understanding.
Comprehension focuses on interpreting content
読解 means reading comprehension. A question about a short passage is one context, but answering that question does not by itself report that every answer was correct.
「物語の読解には時代背景の理解も役立つ。」 (Understanding the historical setting also helps with reading the story.) Understanding the historical setting helps interpretation of this story. It is not a rule that every passage requires knowledge of a particular historical period.
Reading names a book-reading activity
読書 names the activity of reading books. A sentence may focus on spending time reading rather than on how accurately every part of a text was understood.
「寝る前に三十分ほど読書する。」 (I read for about thirty minutes before bed.) The example states a reading activity and about thirty minutes before bed. That duration does not establish complete understanding of the book.
Time spent reading does not measure understanding
Comprehension is not confined to test questions: the story example relates interpretation to background understanding. Likewise, a quiet afternoon is one setting for reading, rather than a condition required by 読書.
Ask what content is being understood and what activity fills the time. Reading books can involve comprehension, but the fact of reading alone does not establish that all the content has been understood.
How do the example about a story's historical setting and the example of thirty minutes of 読書 before bed differ in focus?
Show the answer and why
The first concerns help in interpreting the story's content. The second concerns the activity of reading books and its duration. Do not confine comprehension to tests or turn thirty minutes of reading into a report of complete understanding.