While I cannot think of a direct example or book which has changed me intellectually, I must offer this example.
Both of my children have been avid readers since they learned to read. that being said, both (in testing) have always excelled in language, comprehension, and critical analysis. While I know that they read far more than other students their age, I can only think to attribute their above average testing scores to reading.
Outside of that, one thing I recognize that I do is look at the backward movement of a text after finishing a text. What did I learn? What questions were raised? What new perspective did I learn? I can, therefore, contribute this to "intellectual development."