Barnes & Noble is one place to find a collection of published Penthouse letters. The Letters to Penthouse series includes numerous publications that are subtitled to depict the theme of the letters in each individual publication.
The Letters to Penthouse series are found through the Barnes & Noble web page and local Barnes & Noble stores. The stories that are published in this series are erotic stories that made their original debut in the Penthouse magazine. As of June 2015, Letters to Penthouse XXXVII: Naughty Intentions and Dirty Deeds, published in May of 2010, is the most recent publication in this series.