In my opinion, "A boy didn't read every book"is ambiguous, it can express"a boy didn't read any book" or "Not every book were read by the boy".Am I right?
"He didn't read every book", in almost any context, means he read some of the books but not all of them. For the other meaning you would say "He didn't read any books." or "He didn't read a single book."
He lost every game (=he didn't win any games)
He didn't win every game (=he lost some games)
You're right, it's ambiguous.
It's not a form of speech that would normally be used, but it became somewhat popularized in the book and TV series A Game of Thrones.
In the story, one of the characters, Jaqen H'ghar, is known to express ideas in an odd way, using a form of speech called the third person indefinite.
Here are some lines are from the Fandom Game of Thrones wiki:
"Help was not promised, lovely girl. Only death."
"A man has a thirst. A boy can make a friend."
"A man teaches a girl."
"A man thanks a girl."
"A girl has no name."
The problem with using this form of speech is that, as you point out, the inferred meaning can be ambiguous.
A boy didn't read every book.
At a literal level (in the context of how the speaker is using the phrasing), the meaning is explicit. It's a simple fact that a particular boy didn't read every book—but other details are not provided.
If you assume it to have a different meaning, then you are making assumptions that aren't expressed in the exact wording. But communication doesn't often operate at a literal level, and we do make assumptions based on context.
So, it's broader meaning is unclear:
- Were all the books read by someone else? (Perhaps a girl read every book.)
- Did the boy read some of the books? (Perhaps a boy only read two books.)
If somebody were to say the phrase in question to me, I would immediately reply with, "What do you mean?"
When things are referred to in this manner, many of the normal assumptions we make about what's being expressed can no longer be made. This is partly because people don't normally speak in this way, so we're not used to knowing how to correctly interpret what's being said.