From the mid-17th century until Becquer in the mid-19th century, there truly is a dearth of quality Spanish-language literature of any sort. The Zorrilla came out in the mid-19th century and I seem to recall it being good, but not great (I cited it only as another Don Juan example, not something that stands out on its own). It really wasn't until the Generation of '98 that anything collective of a good sort came out of Spain (Unamuno I think is neglected; Machado, after Darío and Becquer is my favorite Spanish-language poet, but he was in the next generation that included Lorca).

But if you come across Fuentes' book, get it. It's one of those books that connects all sorts of dots when it comes to Spanish/Latin American literature and it isn't polemical or political in nature.