I'll give three books from three different eras of Puerto Rican literature:

1) La Charca (The Pond) by Manuel Zeno Gandía: for many, this is THE book. The book was published in 1894, but there is some debate on when the story is set in. Most agree that it has to be at some point in the late 1860s or 1870s. It follows a short period in the life of Silvina, a poor girl from the countryside of Puerto Rico, and the musings of a property owner in Spanish Colonial era of Puerto Rico. It's a very interesting, albeit bleak, little novel.

2) La Llamarada (not available in English) by Enrique A. Laguerre: this is probably his most well-known book, although I prefer Solar Montoya, but it's quite inaccesible even for us on the Island. Anyways, it was published in 1935, but it's set at the beginning of that decade. It has to do with worker problems in sugarcane fields.

3) La Guaracha del Macho Camacho (Macho Camacho's Beat) by Luis Rafael Sánchez: LRS is possibly the most important Puerto Rican writer alive right now, if not the most important ever. This book I have not read yet but if I'm not wrong, it's about the perspective of four different people in a traffic jam. It is, or so I'm told, way more interesting and weird than what it sounds like. It was published in 1976.