Check out these excellent online resources on the topic of holy souls and purgatory.
Read Me or Rue It: E-book on Purgatory
The Secret of the Poor Souls in Purgatory: An Interview with Maria Simma of Austria
Treatise on Purgatory by St. Catherine of Genoa
Prayer of St. Gertrude the Great for the Souls in Purgatory
(A prayer to release many souls from Purgatory each time it is said and which was extended to include living sinners as well.)
Eternal Father, I offer You the most precious blood of thy Divine Son, Jesus, in union with the Masses said throughout the world today, for all the Holy Souls in Purgatory, for sinners everywhere, for sinners in the universal church, for those in my own home and in my family. Amen.