Álvaro Armando Vasseur's Poemas was the first full-length edition of Whitman's poetry published in Spanish. The first link below is to a critical introduction that provides textual, historical, and biographical contexts for the work. The second link is to the full text of the translation.
In 1951 Vasseur published a preface intended, it seems, for the sixth edition of his book-length translation of Whitman's poetry, Walt Whitman: Poemas. The preface was published in the magazine Alfar, in Galicia--the only place in which the preface would appear. In it he talks extensively about his methods of translation, the history of his version, and the opinions of the philosopher George Santayana about Whitman's work. Most of the translation that appears here was originally published as Matt Cohen and Rachel Price, "Álvaro Armando Vasseur's Preface to the Sixth Edition of Walt Whitman: Poemas," PMLA 123.2 (2008): 438-451. It is republished here with the permission of the Modern Language Association.