Hogwarts: A History is one of Hermione's favorite books and has been for the previous three years that she's been a Hogwarts student. It offers the history of the school and an overview of the various magical rules and enchantments that protect the school. For example, the book informs Hermione that it is impossible to Apparate on school grounds. However, at the beginning of her fourth year, Hermione discovers her beloved reference text leaves a number of things out in its exploration of Hogwarts--namely, that all the domestic labor at the school is performed by house-elves. This impresses on Hermione that she can't believe everything she reads outright; instead, she needs to understand that everything written expresses information from the perspective of the writer, rather than presenting an objective truth. This realization brings about Hermione's intellectual coming-of-age, turning her into an adult capable of thinking critically about what she reads.
Hogwarts: A History Quotes in Harry Potter and the Goblet of Fire
"It's all in Hogwarts: A History. Though, of course, that book's not entirely reliable. A Revised History of Hogwarts would be a more accurate title. Or A Highly Biased and Selective History of Hogwarts, Which Glosses Over the Nastier Aspects of the School."
"What are you on about?" said Ron, though Harry thought he knew what was coming.
"House-elves!" said Hermione, her eyes flashing. "Not once, in over a thousand pages, does Hogwarts: A History mention that we are all colluding in the oppression of a hundred slaves!"