What is a significant feature of the rare books library at the Hispanic Society of America?

The Hispanic Society of America's rare books library is renowned for its extensive collection of historical texts, specifically holding around 15,000 books printed before 1700. This aspect of the collection is significant because it underscores the library's commitment to preserving and showcasing early printed works that are crucial to understanding the cultural and literary history of Spanish and Portuguese-speaking regions. The volume of these early books also reflects the library's mission to promote scholarly research and a deeper appreciation of the Hispanic cultural heritage.

The other options do not accurately represent the library's focus. The number of books printed after 1700 is not as substantial in this collection. A first edition of "Hamlet" would be more aligned with English literature rather than the primary focus of the Hispanic Society, which centers on Hispanic heritage. Finally, specializing in modern literature diverges from its foundational collection of rare and historically significant older texts. Thus, the correct answer highlights the library's unique feature that aligns with its mission and historical significance.