The character who said/thought the following quote, "What do you think about that? As a matter of fact, you needn't ascertain. I ascertained! The books are real!" is Owl Eyes.
Explanation:
1. Owl Eyes is a character in F. Scott Fitzgerald's novel "The Great Gatsby."
2. This quote reflects Owl Eyes' fascination and surprise at finding real books in Gatsby's library, emphasizing the material authenticity of the books.
3. Owl Eyes' attention to detail and realization about the genuine nature of the books demonstrate his character's observational skills and interest in tangible reality within the lavish facade of Gatsby's world.
Therefore, Owl Eyes is the character who expresses this quote in "The Great Gatsby."
