What is the merit of praying for the souls of the faithful in purgatory compared to for the living?

From the preface by the publisher to Rev. Schouppe's *Purgatory Explained by the Lives and Legends of the Saints* : > Our suffrages or offerings for the dead are more agreeable to God than our prayers and good works for the living, for the Poor Souls are closer to God, stand in greater need and cannot help themselves. Is this true? I could not find a quote to back it up in the book.