An English teacher corrected her student when he wrote:
My science textbook says that water is made of hydrogen and oxygen.
She told them that "We read books, they do not speak." So the students had to write:
I read in my science textbook that water is made of hydrogen and oxygen.
The teacher's instruction appears to make sense. Books are not known for making sounds. However the English language does not always make sense. The most common meaning for say / said is a spoken statement. However various dictionaries provide another definition, which is worded like this:
to state an opinion or fact
to indicate, show
English books and articles often use say / said in a general way that is not about speaking. Here are some example sentences where say / said is used in a non-speaking structure:
The clock says fifteen minutes after twelve.
Shakespeare said that alcohol "provokes the desire, but it takes away the performance".
The Oxford English Dictionary says that the word "say" can be used in relation to a book or author.
Most English grammar textbooks do not clearly explain this point. The most prestigious English dictionary in the world is The Oxford English Dictionary, often called the OED. So to settle this issue I got out my copy of the OED (1st edition) and looked up the entry for "say". It treats the word "say" with a lot of detail. It very specifically explains that the word "say" can be used in relation to a book or an author. "Also in wider sense, used of an author or a book with quoted words as object."
It may not appear to make sense, but it is correct and normal English to report that a book says something. (Normally one uses the simple present: The Bible says we must love our neighbours. When referring to an obsolete example, use the simple past: The old law said that a man with a red flag had to walk in front of any moving automobile.) Asking students to correct something that is not wrong makes English more confusing and difficult for them. It is fine to avoid this use of say / said yourself. You don't even need to teach this grammar point as an extra rule. However students will be confused when they read English books that use a structure they were told was a mistake.