El Pais has an interesting list of the important works of the 20th century from Latin America, before the boom. Like any list it is flawed but is an interesting starting point and a nice way to see that there was literature before the Boom.

1918 Los heraldos negros. César Vallejo

Cuentos de la selva. Horacio Quiroga

1920 El hombre muerto. Horacio Quiroga

1922. Trilce. César Vallejo

1924. La vorágine. José Eustasio Rivera

Veinte poemas de amor y una canción desesperada. Pablo Neruda

1926. Don Segundo sombra. Ricardo Güiraldes

El juguete rabioso. Roberto Arlt

Cuentos para una inglesa desesperada. Eduardo Mallea