>But when the interpretation follows a methodical and systematic analysis thorough the book (or all of the books from an author), it could be attributed to the writer even if it wasn't consciously intended.

But was it the book that got you to that particular interpretation or was it the systematic analysis? If the author simply meant to write a fun read with no expectation of deep analysis, then I would say it's your analysis that is creating that meaning for you from the text, not necessarily the text itself.