The Purgatorian Manual is a devotional aid in praying for the Holy Souls. For every day of the month it has a short meditation on some aspect of Purgatory, a related prayer for the Holy Souls, a special intercession for the Holy Souls, and a practice for the day. It was originally published in 1930 by the Redemptorist Fathers. As far as I know it is in the public domain. The original manual had a bunch of extra prayers in addition to the daily entries, but most of those are easy enough to find elsewhere, so I did not reproduce them here.
I have found this to be an outstanding resource, and as the Holy Souls are so neglected today, I wanted to make this more widely available.
UPDATE 11/29/2012 Vigil of St. Andrew: This is now available in epub and kindle ebook format!